I had this problem immediately after signing in this morning. CPU utilization (the usual cause of slow performance on my old machine) was fairly low but disk active time was a solid 100% for several minutes. This has been happening to me at startup for several months, but not every time. It seems to correlate with whether I leave my OneNote window open while putting the PC to sleep for the night, and the duration of the high-use event seems to correlate with how long it's been since I last closed OneNote. I keep 2 Task View desktops open all the time and usually keep OneNote loaded in one of them, and I rarely sign out of my Windows account at all
When this happens, I notice that it isn't just slow after I sign in to my Windows user account. There's also a much longer delay between the appearance of my lock screen background and the presentation of my pin entry field, if I hit the spacebar to open the sign on screen as soon as the lock screen appears.
Possible workaround
Since (in my case at least) this seems to be related to leaving apps open during sleep, a workaround that might help is to make sure all your app windows are closed before sleeping the PC. I have tried this a few times after particularly slow startup episodes, and it seems to make the next startup happen at normal speed, but I haven't tried it enough times to be 100% convinced that it's a reliable workaround.
The nice thing about this workaround for others is that it's easy and doesn't disturb any settings, so if it helps you're good, and if it doesn't you're no worse off.
Possible cause
The apparent relationship to Task View makes me suspect that the Windows Timeline feature is responsible. When you launch your user account, Task View builds a history of the documents and web pages you've accessed in the last 30 days. Obviously, it must be scanning a journal on disk to build this. It might be that the scan always continues for the full life history of any tasks that exist when the scan started. For apps you leave running across PC sleeps, that could make it scan back much, much farther and could be causing this problem. I'm just speculating, but it would explain why the workaround succeeds, if in fact it does.
I read recently that Timeline will no longer be available in Windows 11, so this problem might go away when you update. If, like me, you don't have a PC that's capable of running Win11, you might be stuck with using this workaround until Win10 support ends in about 4–5 years.