I added 5% grain to a layer on a video, and the video became impossible to work with, every time I try and watch it in premiere pro, it's just sound, as the video is so laggy. I'm running an i9 9900k @5ghz RTX 3090 and 64gig ram.
It's a very processor intensive effect. I have a similar system to yours and the cpu usage goes to 100% when playing. You might have to render the timeline but that also will take a long time, my system was rendering at about 2 fps.