Raptr may be related to some people's issues, but it is definitely not the definitive problem. Though I'd try it out, because I've been experiencing the same problem - I assumed it was something wrong with my computer, and then started investigating - and so I tried the exit game, close Raptr, launch game gimmick, and I was getting the lag issue again.

The only solution that fixed it was by switching the resolution a bunch of times from Windowed Borderless to windowed, then full screen, then back, which completed freed up the problem. Since Raptr was not running at all, it is clearly an issue with the client somewhere.