Why does everything slow down after a Windows update?

After a large update, Windows spends a while reindexing, recompiling, and reoptimizing in the background. On an older machine this can take an hour or more, during which everything feels slow. Restarting repeatedly during that window makes it take longer, not shorter.

So the first thing to try is leaving it alone. Let it sit powered on and idle for an hour, then restart once, cleanly.

If it is still slow the next day, that is a real problem rather than the update settling, and the usual causes are a driver that the update replaced with a generic version, particularly graphics or storage drivers, or an older machine that has genuinely run out of headroom for the current version of Windows.

If it happened across many machines at once, tell us rather than working around it. A bad update reaching your whole fleet is something we would want to catch and, where necessary, roll back. For managed clients this is the reason we test updates on a small group before releasing them more widely rather than letting Microsoft decide the schedule for you.