In September, something similar always occurs: a new version of macOS is released, people install it during their lunch break, and by mid-afternoon their MacBook is "basically dead". It isn't; it's either busy or full.
The good news is that the slowdowns that occur after the update are due to a small number of causes, and most of them can be repaired within one evening. The following explains why this happens, how to fix it, and which tools will speed up the process.
Why a Mac slows down after a macOS update
Background indexing. Immediately after a major update, including Golden Gate, the system runs several background tasks. Spotlight deletes its search index and then creates a new one. Photos goes through the library again to detect faces and pets. Time Machine asks for a significantly larger backup than normal. iCloud also rechecks all the data. None of these operations show a progress bar; they simply use CPU and disk resources for a period ranging from one hour to a couple of days.
Less available storage space. While the update takes up only a few gigabytes, the installer also leaves behind snapshots and caches, and the new system files are larger than the previous ones. A drive that was 82% full can end up at 94% full even if nothing else changes. This matters because macOS uses free disk space as overflow when RAM is exhausted. Without space to swap, everything slows down.
The login items that have returned. Updates tend to re-enable helper apps that were switched off many years ago, such as printer utilities, a forgotten cloud drive, and a menu bar tool from a Kickstarter gadget.
Apps that have not yet caught up with the change. A major macOS update makes changes under the surface. Third-party apps that have not released an update may leak memory or crank a CPU core to 100%, which is exactly how the whole system can feel slow. Browser extensions are typically the cause.
Memory demands have increased. Every new version requires a small amount more RAM. 8 GB was sufficient in 2021 but is now quite tight, and you can't add memory to a current Mac.
In the hardware era, Apple provides a Mac with about seven years' worth of new macOS versions, and the most recent one or two always place the highest demand on the machine.
How to fix it, in order
1. Wait one day, plug the Mac in, keep the lid open, and leave it on Wi-Fi all night. In about half of the cases where people complain that their Mac is ruined, the problem sorts itself out by the following afternoon. To make sure it's indexing rather than having frozen, open Activity Monitor (under Applications > Utilities, or by pressing Command-Space and then typing it), click on the CPU tab and sort the entries by % CPU. If mds_stores, mdworker, or photoanalysisd are at the top, then it's indexing, and you should leave it alone.
2. Free up storage space. Free up space by going to System Settings > General > Storage and making sure you have at least 10 to 15% of storage space left. Although the Recommendations panel functions, it does so slowly. Better solutions include emptying the Trash, deleting old iPhone backups, and clearing the Downloads folder, since on most Macs it contains many old .dmg files.
3. Remove the login items. Go to System Settings > General > Login Items & Extensions and, under Open at Login, take out any items you do not recognize. Also, under Allow in the Background, turn off anything from a company that is irrelevant to you.
4. Make sure that all the apps are up to date by running the App Store updates, and then open each non-App Store app that you use every day and check for updates in its menu; if Safari or Chrome is the one that seems slow, turn off the browser extensions one at a time.
5. Check the level of memory pressure. Go back to Activity Monitor, select the Memory tab and look at the Memory Pressure graph at the bottom. Green indicates that everything is okay. Yellow shows that the system is managing things, just about. Red means that constant swapping to the disk is taking place. If the level is yellow or red even though only the browser and Mail are open, then the solution lies in changing your behavior: open fewer tabs, quit applications rather than hiding them, and be aware of Slack, Teams and Discord, all of which will consume a gigabyte each throughout the day.
Before going any further, it's worth asking whether the Mac is a good fit for this release. Apple's compatibility list says whether a model can install macOS 27, not whether it will run it comfortably. There's software to check it and upgrade that reports what a given model actually supports and whether the jump is worth it. If the answer is no, the two steps below won't change much.
6. Safe mode. To enter it, shut down the machine, hold the power button until the startup options appear, select the disk, then press Shift and click Continue in Safe Mode. This clears certain caches and bypasses third-party login items. After that, restart the computer normally.
7. Reinstall macOS using the Recovery system. It doesn't sound all that drastic, since it rewrites the system files but leaves your personal data untouched. If an update has been interrupted partway through, it can leave behind a slightly damaged installation which no other method can repair; reinstalling the same version will fix this. Nevertheless, back up the data first.
Tools that can speed things up
Built-in tools: Activity Monitor to spot runaway processes, the Storage pane to find what's eating the disk, and Disk Utility's First Aid to check the drive after a rough update.
CleanMyMac (by MacPaw) brings most of the steps mentioned above together in one location: it removes system junk, caches and leftover installer files, lists the login items and background agents so that they can all be switched off at once, uninstalls programs completely rather than leaving behind fragments, and also provides a malware scan. It is the quickest way for a person who does not want to go through System Settings.
AppCleaner (free) excels at one task—when you drag an app onto it, it deletes the preference files and the support folders that Finder would otherwise leave behind.
OnyX (free) is a traditional maintenance utility you can use to rebuild caches and run system scripts. It's powerful, but it's best left to those comfortable working with the system's internal components.
Sometimes the honest answer is the hardware
Golden Gate has completely dropped support for Intel Macs; a 2020 Intel MacBook running the final Tahoe update will operate more slowly than it did on Big Sur, and no improvements will offset that. Occasionally, the best course of action is to stay on the version that performs well, keep receiving security updates, and replace the machine on a schedule decided in advance rather than waiting for an update.