macOS High Sierra Running Slow? The Ultimate Guide to Diagnosing and Fixing Your Mac‘s Speed Issues

When Apple released macOS 10.13 High Sierra in September 2017, it promised a range of performance improvements and new features. However, for many users, the excitement quickly turned to frustration as they encountered a variety of slowdowns and compatibility issues after upgrading.

In a survey of 5,000 MacRumors readers, over 37% reported that their Macs felt slower after installing High Sierra. Complaints about long installation times, sluggish performance, overheating, and app crashes flooded Apple‘s support forums.

As a Mac software expert who‘s helped countless clients optimize their machines, I‘ve seen firsthand how disruptive a botched macOS upgrade can be. In this in-depth guide, I‘ll share my proven strategies for diagnosing and resolving the most common High Sierra speed issues.

Understanding High Sierra‘s Under-the-Hood Changes

To troubleshoot effectively, it helps to know what‘s going on beneath High Sierra‘s sleek interface. The update introduced several significant changes to the Mac‘s core technologies:

  • APFS (Apple File System): High Sierra automatically converted many Macs‘ SSDs to APFS, a new file system optimized for flash storage. However, the transition didn‘t go smoothly for everyone, with some users reporting slower file transfers and Time Machine backups.

  • HEVC and HEIF: High Sierra added support for HEVC (H.265) video encoding and HEIF image compression, which can reduce file sizes by up to 50%. However, this also means the CPU has to work harder to encode and decode media files, which can slow down performance on older Macs.

  • Metal 2: The update to Apple‘s graphics API promised better performance and compatibility with external GPUs. But it also caused issues with some older graphics cards and led to compatibility problems with certain apps.

With these changes in mind, let‘s dive into diagnosing and fixing the most common High Sierra speed issues.

Issue 1: Painfully Slow High Sierra Installation

If you haven‘t yet taken the plunge to High Sierra, you might be in for a long wait. Many users reported installation times of 2-3 hours or more, even on relatively new Macs.

Possible causes:

  • Insufficient free disk space
  • Outdated third-party drivers or kernel extensions
  • Partially downloaded or corrupted installer files


  1. Make sure you have at least 20GB of free space on your startup drive. If not, delete some large files or move them to an external drive. You can use a disk space analyzer like DaisyDisk to quickly find and remove space hogs.

  2. Disconnect any non-essential peripherals, especially those with third-party drivers (e.g. graphics tablets, audio interfaces). Kernel extensions that haven‘t been updated for High Sierra can cause the installer to hang.

  3. Go to the Applications folder and delete the "Install macOS High Sierra" app if you‘ve already downloaded it. Empty your Trash, then reboot your Mac and re-download the installer from the App Store. A fresh copy of the installer is less likely to be corrupted.

If you‘re still seeing extremely slow progress after an hour, you might need to create a bootable High Sierra installer on a USB drive and use that to perform a clean install instead.

Issue 2: Sluggish Post-Install Performance

Once you‘ve made it through the High Sierra installation gauntlet, you might breathe a sigh of relief—until you realize your Mac feels slower than it did before the update.

Possible causes:

  • Spotlight re-indexing your drive
  • Photos app analyzing and kategorizing your images
  • Incompatible login items and launch agents
  • Outdated app versions that haven‘t been optimized for High Sierra


  1. Give it time. Right after installation, High Sierra has a lot of housekeeping to do, including re-indexing your files for Spotlight search and analyzing your photos library. This can take hours, even on a fast Mac. Check the progress by clicking the Spotlight icon in the top right corner of your screen – if it says "Indexing," it‘s still working. Similarly, open the Photos app and check if it says "Analyzing Photos" at the bottom of the window.

  2. Restart your Mac in Safe Mode by holding down the Shift key while it boots. This will temporarily disable login items and launch agents and run some system checks. If your Mac feels faster in Safe Mode, it‘s likely that some startup processes were causing the slowdowns.

  3. To figure out which login items might be the culprits, go to System Preferences > Users & Groups > Login Items. Remove any non-essential apps, especially those that you don‘t recognize.

  4. Update your apps to the latest High Sierra-compatible versions. Check the Mac App Store and the developers‘ websites for updates, paying special attention to apps you use frequently.

Here‘s a table of some popular apps that had High Sierra compatibility issues, along with the version that fixed them:

App High Sierra-Compatible Version
Office 2016 15.35
Adobe Photoshop CC 18.1.0
Dropbox 39.4.49
Google Chrome 62.0.3202.89
VirtualBox 5.1.30

Issue 3: Frequent App Crashes and Freezes

High Sierra‘s core tech changes meant that many apps needed updates to maintain compatibility. Without those updates, you might experience frequent crashes, freezes, or errors when trying to launch the app.

Possible causes:

  • Outdated app versions
  • Corrupted app preferences or caches
  • Insufficient RAM for demanding apps
  • Disk permissions issues


  1. Make sure you‘ve installed the latest versions of any crashing apps. For App Store apps, open the App Store and click the Updates tab. For non-App Store apps, check the developer‘s website or the app‘s built-in update feature.

  2. If an app still crashes after updating, try deleting its preferences and cache files:

    • Quit the app
    • Go to Finder and select Go > Go to Folder
    • Type ~/Library/Preferences and click Go
    • Find any files with the app‘s name in them, drag them to the desktop, then relaunch the app
    • If that doesn‘t help, repeat steps 1-4 with ~/Library/Caches and ~/Library/Application Support
  3. Open Activity Monitor (in Applications > Utilities) and click the Memory tab. If an app is using a large amount of RAM (several gigabytes), close any other non-essential apps and browser tabs to free up memory.

  4. If you‘re still seeing frequent crashes, your disk permissions might be damaged. To repair them:

    • Restart your Mac in Recovery Mode by holding Command+R while it boots
    • In the Recovery window, select Disk Utility and click Continue
    • Choose your startup disk on the left, then click the First Aid button
    • Click Run to start the repair process

Issue 4: High CPU Usage and Overheating

Is your Mac‘s fan kicking into overdrive more often after installing High Sierra? This could indicate that certain processes are hogging your CPU and generating excess heat.

Possible causes:

  • Spotlight indexing or Photos analysis still in progress
  • Safari content blocker or ad blocker using significant CPU resources
  • Runaway processes or memory leaks from poorly optimized apps
  • SMC settings need a reset


  1. Open Activity Monitor and click the CPU tab. If you see processes like photoanalysisd or mds using a large percentage of CPU, just wait – these are the Photos analysis and Spotlight indexing processes, respectively, and they‘ll eventually finish.

  2. If you‘re using a content blocker extension in Safari, try disabling it temporarily. Some poorly written blockers can consume a lot of CPU power. I recommend using a lightweight, native content blocker like 1Blocker instead.

  3. Look for apps or processes in Activity Monitor that are consistently using over 70% CPU, even when you‘re not actively using them. If they don‘t settle down after a minute or two, select them and click the X button to force quit them.

  4. Reset your Mac‘s SMC to restore the default heat and fan control settings. For a MacBook:

    • Shut down your Mac and plug in the power adapter
    • Press Shift+Control+Option on the left side of the keyboard, then press the power button
    • Hold all four keys for 10 seconds, then release them and press the power button to turn your Mac back on

After resetting the SMC, monitor your CPU usage and temperature for a few hours to see if the issue is resolved. If you‘re still seeing high temperatures, you might need to open up your Mac and clean out its fans and heatsinks, or reapply thermal paste to the CPU – bring it to an Apple Store or reputable repair shop if you‘re not comfortable doing this yourself.

Issue 5: Slow Startup and Login Times

Nobody likes waiting around for their Mac to boot up. If your startup times have gotten noticeably longer after upgrading to High Sierra, there are a few likely culprits.

Possible causes:

  • Too many login items and launch agents
  • Disk errors or file system corruption
  • Outdated firmware on an SSD
  • Hardware issues (e.g. failing hard drive)


  1. Disable unnecessary login items by going to System Preferences > Users & Groups > Login Items. Uncheck the box next to each app you don‘t need to launch at startup.

  2. Run First Aid on your startup disk using the steps from Issue 3, Fix 4 above. If it finds and repairs any disk errors, restart your Mac and see if startup times improve.

  3. For Macs with third-party SSDs, make sure you‘ve installed the latest firmware update from the manufacturer‘s website. Outdated firmware can cause slowdowns and compatibility issues.

  4. If you‘re still experiencing long startup times (over 1 minute), you might have a hardware issue, like a failing hard drive or insufficient RAM. Run Apple Diagnostics by holding down the D key while your Mac boots. If it reports any hardware errors, take your Mac to an Apple Store or authorized repair shop for further diagnosis.

Bonus Tips for Boosting High Sierra Performance

Beyond fixing specific issues, there are some general best practices that can help keep your Mac running smoothly on High Sierra:

  • Do a clean install: If you‘re upgrading from an earlier version of macOS and your Mac is feeling sluggish, consider doing a clean install instead of an in-place upgrade. This wipes your startup drive and gives you a fresh start without any old junk clogging up the works. Just make sure to back up your important data first!

  • Use an SSD: If your Mac doesn‘t have a solid-state drive yet, it‘s the single best upgrade you can make for overall performance. SSDs are much faster than traditional hard drives and can breathe new life into an older Mac. See our guide to upgrading your Mac to an SSD for step-by-step instructions.

  • Manage your startup items: Every app that launches at login increases your Mac‘s startup time and consumes valuable CPU and RAM. Open System Preferences > Users & Groups > Login Items and ruthlessly prune the list to just the essentials.

  • Keep 10-20% of your storage free: macOS needs some breathing room to function at its best. If you‘re running out of space on your startup disk, your Mac might slow down or even refuse to boot. Open About This Mac > Storage to check your free space, and aim to keep at least 10-20% of your total storage capacity available.

Here‘s a table of some additional recommended speed-boosting tweaks, ranked by effectiveness:

Tweak Effectiveness
Use an SSD ⭐⭐⭐⭐⭐
Keep 10-20% of storage free ⭐⭐⭐⭐
Disable login items ⭐⭐⭐
Disable transparency effects ⭐⭐
Clean up launch agents

The Bottom Line

While High Sierra brought some great new features and under-the-hood improvements to macOS, it also introduced its fair share of performance issues and compatibility headaches.

If you‘ve been battling slow speeds, app crashes, or overheating since upgrading, I hope this guide has given you some practical tools to diagnose and resolve those problems.

Remember, even if you follow all the tips and tweaks in the world, your Mac will eventually start to feel slower as newer versions of macOS and more demanding apps are released. High Sierra was a turning point for many older Macs – it pushed their hardware to the limit and exposed underlying issues.

If you‘re still having trouble getting High Sierra to run smoothly on your Mac, it might be time to consider upgrading to a newer model. The performance gains from a faster CPU, more RAM, and speedy SSD storage can be truly transformative – it‘s like getting a brand new computer.

At the end of the day, the best way to keep your Mac running at peak performance is to stay on top of regular maintenance tasks like freeing up disk space, keeping your software up to date, and monitoring resource usage in Activity Monitor. A little proactive TLC can go a long way towards extending your Mac‘s lifespan and saving you from pulling your hair out over slow speeds.

Here‘s to a future of fast, reliable Macs – whether you‘re rocking a vintage model or the latest and greatest from Cupertino! May your High Sierra woes be short-lived and your productivity unbounded.

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