@UserNoah 10bit output was unrelated.
I found the cause. It is
"Use Float16 Textures". (Preferences>Tweaks>GPU Usage(change from OpenGL?))
Turning it off solved the problem. Turning it on will reduce all UI performance during RAM preview.
— Use Float16 Textures: If your graphics hardware supports 16-bit floating-point textures, activating
this option will force int16 and float32 images to be uploaded to the viewer as float16 instead, which
may improve playback performance.
My 4k footage is int16. So I added ChangeDepth node and converted it to float16. Performance is back!!
"Use Float16 Textures" of Fusion Studio v18.5b2 with Nvidia 528.95 is not working.