Why your Mac slows down right after an update
You’ve just installed the latest version of macOS, restarted, and now everything feels like it’s wading through treacle. Apps bounce in the Dock for ages, Spotlight searches stall, and the fans spin up over nothing. This is one of the most common complaints we hear at the workshop, and the frustrating part is that the answer is usually “it depends”.
Some of the slowdown is completely normal and temporary. Some of it is a genuine software conflict you can fix yourself. And occasionally, the update is simply the moment your Mac’s ageing RAM or drive stops coping. Knowing which one you’re dealing with saves you a lot of pointless fiddling.
Give it a day: background indexing
The single most common cause of post-update sluggishness is Spotlight re-indexing. After a major macOS update, your Mac rebuilds its search index from scratch — cataloguing every file, email and message on the drive. This runs in the background, hammers the CPU and disk, and can carry on for several hours after you think the update has finished.
Photos does something similar, re-analysing your library for faces and objects. iCloud may re-sync. Time Machine often kicks off a fresh backup. All at once, on a machine you’ve just told to work hard.
The honest advice here is to leave the Mac plugged in, awake, and largely alone for a few hours — ideally overnight. Click the Spotlight magnifying glass; if it says “Indexing…”, that’s your culprit. If the Mac is still crawling two or three days later, the problem is something else.
Check your storage pressure
macOS behaves very differently when the drive is nearly full. Once free space drops below roughly 10–15%, the system loses the room it needs for swap files and temporary caches, and everything grinds. A macOS update makes this worse in the short term because it keeps the old system files around until it’s confident the new install is stable — so you can temporarily lose tens of gigabytes.
Go to the Apple menu → System Settings → General → Storage and look at how much is genuinely free. If you’re scraping the bottom of the drive, clear some space: empty the Trash, offload large video files, and check the Developer and Caches categories. Freeing up headroom often restores responsiveness on its own.
Track down incompatible apps
A new macOS version can leave older software stranded. The usual offenders are anything that hooks deep into the system: outdated antivirus tools, VPN clients, backup utilities, disk cleaners and menu-bar helpers written for a previous macOS generation. These can sit in the background retrying, crashing and restarting, quietly eating CPU.
Open Activity Monitor (Applications → Utilities), sort by % CPU, and watch for a couple of minutes. Anything consistently near the top that isn’t a task you started is worth investigating. Check the developer’s site for an update, and remove anything you no longer use. Kernel extensions from old software are a particularly common cause of beachballs after an upgrade.
When RAM is the real bottleneck
Here’s the part people don’t want to hear: sometimes the update didn’t break anything — it just raised the baseline. Each macOS version tends to assume a little more memory than the last. A Mac with 8GB of RAM that felt fine two versions ago can start swapping to disk constantly under a newer, hungrier system.
In Activity Monitor, open the Memory tab and look at Memory Pressure at the bottom. If that graph sits amber or red during normal use, your Mac is out of RAM and leaning on the drive to compensate — which is slow. On older Intel MacBooks the memory can sometimes be upgraded; on Apple Silicon Macs it’s soldered, so the fix is different. Either way, it’s worth diagnosing properly rather than guessing.
When the SSD is on its way out
The other hardware culprit is the drive itself. A large macOS update writes enormous amounts of data, and that sustained load can expose an SSD that was already degrading. Symptoms include long stalls, apps refusing to open, and occasional freezes that clear after a reboot.
If your Mac has a slow, ageing hard drive rather than an SSD, an SSD upgrade is genuinely the single biggest speed improvement you can make — often more transformative than a new machine. On models where the drive is replaceable, this is a straightforward job. Where it’s soldered, we’ll tell you honestly what the options are.
Get it diagnosed properly
If you’ve waited out the indexing, cleared storage space, and tidied up old apps but your Mac is still sluggish, it’s time to find out whether you’re looking at a software conflict or a hardware limit. A proper health check checks the drive’s health, memory pressure and running processes, so you’re not throwing money at the wrong problem.
We handle MacBook repair — including SSD replacement, memory upgrades where possible, and honest advice on when a Mac is worth keeping — at our Putney workshop. Call 020 7610 0500, drop us a message on WhatsApp, or use the contact form and we’ll take a proper look before recommending anything.
