I have the same issue. I even tried SH3D v4.6-portable with Java1.6 and the same issue is there.

However, I think it is bound to the high monitor resolution (and some inefficient rendering logic used in SH3D).
I have an old PC Win10 with 1280x800 resolution and SH3D works smoothly on it.
On my standard PC Win10 with 4K display - it is unusable.

However, I found that if I switch "Room rendering in plan" to be "Monochrome" (in File/Preferences menu) - it starts working smoothly. So I consider this as a workaround.