@verydima - Do you have "limit bandwidth" checked? If not, make sure you have it enabled. Although that shouldn't make a difference if there are no projects running.
Have a look at a similar thread i answered in 2020 -- https://forum.gsa-online.de/discussion/comment/167555/#Comment_167555
It looks like I found my error.
I try to check all registered domains in different zones.
Today I found my IP in Spamhaus with the description "....Troyan... "
I installed Malwarebytes, and now I see a lot of messages about blocked URLs.
And I have much less CPU load.