I did some more tests.
I focused on uwxp as the begin and i had this results:
1. not plane with engine on, cloudy day 45 fps i have stutters
2. switches off uwxp: fps aroun 55, stutters disappeared but started again after around 20 seconds (fps still the same)
3. switch on again uwxp: fps again aroun 40-45, stutters disappeared but started again after around 20 seconds, fps 40
looking at the "performance cake", it's like after a while, the m20 plugin starts from 0% of resources (when there is no stuttering) going to 33% and blinking every second from 33% to 17%