Ok, I found something. I turned minimap label off (minimap.xc->labelsEnable->false)
I got clear instant fps buff (more stable)
After 15 games, I got no fps drop !
This might not be a universal solution and still need more test.
but anyone who is facing fps issue should give it a try.
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this is weird because, last year, I excluded minimap from being the cause.
at that time I turned minimap off in aslain's installer. this time I install xvm manually and turn map label off in config.