Sorry but without a lot more detail and even then I think you should check out what others have written at https://www.google.com/search?q=rainbow+6+siege&ie=utf-8&oe=utf-8#q=rainbow+6+siege+fps+drop
Start with the ones that folk solved it.
PS. If you post a Web Speccy, I'll give it a going over, sharing what I'd do if this was my PC.
Pc build with specs--->https://pcpartpicker.com/b/Nn7WGX
SO i recently built my first gaming pc. Everything will run maxed out but all of a sudden a few days ago, rainbow 6 siege as well as a couple other games arent working as well. Instead of running anywhere from 100-144 fps they have dropped to 25-40. But then if I go and open up Doom or BF1 it runs perfectly fine. Its really starting to get on my nerves. I dont really think its the video card itself if its running other games okay. Im assuming it may be drivers but I dont really know what would be causing it. Its not my display adapter or my video card drivers in general I dont think because ive already tried multiple drivers. If anybody could give some input that would be awesome.