I was updating, or basically finishing bigger sets like MSX, MSX2 and ZX Spectrum, which involved adding considerate amounts of files in one go, although I only did add them letter by letter.
Even though the process of adding and downloading media was finished, it kept my PC very slow, I didn't check HDD I/Os. Just strange that after the whole process the PC was still "recovering". The moment I closed LB, after 1-2 minutes, everything was running normal again.
I tried now adding around 10 missing PSX games and it didn't happen then.
The speed within LB itself increased drastically, but not only since .NET 6, already before, adding games is very fast, I mean adding 100 games is done in seconds and the downloading media process after that, like downloading 1.000 images is also done in 1 minute or less.