I'll assume 1 or 2 things here; either you play with a controller or keyboard turn, or you just happen to have a very strong constitution and you never get motion sickness.
If you don't use the mouse to turn and aim, then just realize that it's very different when played this way. If you do, consider yourself lucky.
I don't use a controller nor keyboard turn. I've used a Mouse to control my camera on every game on my PC since Doom added Mouse support.
Many people, myself included, get motion sickness with mouse aiming when the FPS are below 40. Even at 40 I'd have to limit myself to short periods of time. At 50+ I can play a game over an hour when using mouse turning/aiming. This is why I have a very powerful computer, so I can maintain over 40 FPS on any game I play.
If you're getting sick at 40 or lower, you'll get sick at 41 or higher. For whatever reason, it might take longer, however you'll still get sick eventually. Motion Sickness doesn't magically go away because it's slightly faster at refreshing. I know this because my brother has motion sickness issues like you explain, and it doesn't matter if the FPS is 15, 20, or 120, he gets sick if he plays it for too long. Period.
It is unplayable when you find yourself having to throw up or go lay down for a while due to playing a video game over 30 mins.
The framerate being lower doesn't magically induce sickness while it being higher doesn't. I'll buy the eye-strain deal, but not movement sickness.
