If you are suffering low frames per second for Rainbow Six Siege here are some things you could try to improve this.
Make sure it is running your card and not your CPU
Right-click on the games .exe and run using your nvidia processor.
You can change this to run GPU by default by going into Nvidia's control panel and finding the "Set PhysX Configuration" and make it select GPU rather than auto select.
Process is likely similar if you have an AMD
Credit: https://steamcommunity.com/app/359550/discussions/0/523898291498535706/
Ensure your drivers are up to date
Credit: https://steamcommunity.com/app/359550/discussions/0/350542145698667359/
Another thread on it: http://www.tomshardware.co.uk/answers/id-3618417/low-fps-siege.html
The game by defualt maxes out the settings to what it thinks your pc can handle, I tweaked mine down as I didn't get the fps that I wanted. Mine sits at 120/140.
If you are still having trouble post let us know.