Sometimes a plugin can heavily downgrade the speed: ReSharper, Visual Assist, TabNine, etc.
If you disable Intellise VS is still slow?
Is there any running process in the background?
Try to remove .vs, Binaries, Intermediate, Saved directories and .sln file then rebuild your project.
I think this isn’t related to UE according to these threads:
(My advice instead of VS: Rider. 
)