Recently tried to re-play HL2. Found it was highly motion sickness inducing, possibly due to unnaturally fast change in perspective when turning. Had to give up very soon. Newer games tend to fare better in that respect.
As a person who almost can't play any 3D first-perspective (shooter, action) games due to serious motion sickness, I'd say a slower cursor doesn't really make much difference. If you're not used to it (due to being different from your desktop environment), it probably would make it worse.
To me, the biggest factors to (worse) sickness are
1. Narrow FOV: usually the bigger the FOV, the better. But when it starts to have too much of "fisheye" effect, it can be detrimental.
2. How "fluent" your character moves -- if you have to stop/decelerate constantly (for any reason: interact with objects, clash with walls, sharp turns), it induces sickness very quickly.
Third perspective games are typically much better, but if it's in a confined space (like in a room) for extended time, it's as bad as FPS.
The best FPS I've played in term of having close to no motion sickness is Overwatch. Apex Legends is pretty good in this regard too.
Interesting, I have never had any problem with Source-based games.
The worst offender for me was Talos Principle and Far Cry 4 due to the headbob effect which couldnt be turned off (why?!).
Portal 1 is the game that made me realize I get motion sickness from first person games. It really caught me by surprise. Amazing game though, 10/10 would do it again.
Couldn't you just move the mouse slower so the turning is slower? Or lower the mouse sensitivity?