So I tried a fresh 9.2.3 installation with default settings and VC-1 plays smooth 
Then I went one by one through all video settings and found one which causes stuttering - whitelist for display mode. For me it was set according to my TV to one value only 3840x2160 60Hz. I remember selecting explicitly this in whitelist on some rather earlier coreelec version, because I had display flickering without it while playing some 4K videos. Now I cleared whitelist and all is fine.
So can you give a short description of this whitelist setting? Surprisingly it affected only VC-1.