I'm running BOINC with yoyo@home and distributed.net OGR-28 project on an AMD Ryzen 7 4700U. Following the Windows 10 Update on April 14 (KB5001330), average credit fell from 9000 to 6400 (and still drops). Unfortunately, I can no longer uninstall it.

Has anyone else experienced similar problems?

Thanks and best regards
Wolfgang

First question: Does your RAC fall come from 'less credit per task' or 'fewer tasks completed'? Without details like that, we don't have a clue where to start looking.

The work units have different sizes, so number of tasks completed varies (and I don't know where to find this number anyway). On the project site I can see that the number of GNodes computed has gone down from between 50 to 60.000 to around 34.000, so it must be 'less work completed'. Also, I've checked that all work units returned were successful and claimed credit was also granted. That's for two computers, as detailed below.

Laptop 1 (old): Intel i7 2720QM (running 24/7, same OS as new laptop, different security software); RAC previously around 6340, now 6240
Laptop 2 (new): AMD Ryzen 7 4700U (running 24/7, partly used for office work); RAC previously 9000, then 8400 rising to 8600, now 6400 and falling

Something happened on April 1, which caused a small decrease in average credit (a few 100s) on both machines (the new one seemed to recover, although not completely; the old one settled lower). The big drop on April 14 affected only the new laptop, and I cannot remember having installed or changed anything that could explain it (except for the Win 10 update). I've run a benchmark outside BOINC for which I have data to compare and found no difference in raw performance. It's weird.

I may have an idea what's behind the problem. Possibly it has to do with energy settings and standby mode. It will take a day to see whether changing the settings will have an effect. Would be strange, though.

Known performance regression with the latest update.
https://www.extremetech.com/gaming/322040-a-new-windows-10-update-is-wrecking-game-performance

Thanks to all contributors, I consider the problem to be solved. It was, for the most part, in front of the screen ...

Somehow the energy setting for standby mode got reset from "never" to "30 minutes", so the computer went to sleep during the night (and now and then, during the day). I use a docking station and an external screen (which also has a sleep mode) and totally failed to realize that I was not only waking up the screen, but the computer (there is hardly any fan noise and wakeup is really fast). RAC is on the rise again.

There might be a fix for the borked Win Update:
Windows Update May Solve High CPU Usage Bug

The main point of interest in KB5001391 is a fix for a memory problem. Microsoft hasn't made things entirely clear but it appears it's targeted at recent reports of users suddenly seeing their CPU usage rocket to the point that their computer slows down or even becomes unresponsive. (Source: pcgamer.com)