Windows vm's getting very slow, some RTA tests getting terminated by 8 h timeout

Recently Windows10 x64 and x86 machines at least on branches 5.14 and 5.15 have been getting very slow without any obvious reason (at least what hehalmet has been investigating). RTA has Qt source package builds which usually take 3,5-5 h to run, but lately the tests have been terminated at the 8 hours timeout because the build has been so slow.

Also some IFW tests where installer is run multiple times doing basic installation with different accounts is working very slow so that 8 hours timeout is reached before test gets finished (normally takes about 2h 30 mins, last succesfull run on normal speed 28.2.20).

Slowness seems to have started around 5.5.20. Since then tests can sometimes finish inside the 8 hours but even then they take twice the time they usually do, but rough estimate is that 80% of the tests fail due the slowness.