Hello @XakepSDK, this reads like https://userbase.kde.org/Desktop_Effects_Performance#Suspend_desktop_effects_for_fullscreen_windows behaving as intended and not a Steam-specific issue. A slow transition sounds like a window manager issue and should be mentioned to the kwin devs.
Closing because this does not appear to be a Steam issue.
Nothing extracted yet.
Your system information
Watching videos in full screen disables compositor and because of this going full screen is slow until compositor disables. Turning off full screen is also slow because steam waits until compositor enabled.