Hello,
When I launch the current version of FDM, I see a brief 2 seconds where the interface is white then you see the actual interface on Windows 10. Does anyone else notice that or is it just me? I don't recall see it when I previously used version 5.

perfectly normal, mine even take 10+ second ish after gaming session or after using any application that use a lot of ram on my potato(from 10 year ago)...if u open task manager n see fdm process, normally it would take 50mb+ of ram usage, if it lower than that, lets say 12mb-ish then the white screen will be there for a while waiting for the ui to load.

(correct me if im wrong, im not programmer, just recalling bits from what i read in forum before)also they use cef framework on fdm 5 & qt for fdm6(for cross-platform purpose?)

Both FDM 5 and 6 use multiplatform QT framework, which is in some way system independent. Some UI elements may be redrawn without full hardware acceleration provided by system dependent hardware drivers and some UI slowdown may be seen when the system is heavily loaded.
However, no one can give more accurate hints/suggestions/explanations without knowing more detailed problem description. Follow the reporting rules, please.