Rakly3
Not really Video FPS has nothing to do with tracking FPS, otherwise 30FPS and even 60FPS videos would be unwatchable for the majority of people.
You can pause a video and still move your head around in the scene although at this point you have 0FPS from the video.
90Hz tracking is enough for about 95% of the population to not get motion sickness from it.
Higher FPS either gives you more clarity or less stuttering or a combination of both (depending on the shutterspeeds used to create the video) when there is motion in the video, not motion from your head.
But the thing is, for a good result you have to shoot the video optimized for one framerate, you can not really do multiple at the same time.
Getting lower framerates then you actually shot can be done by dropping frames, which leads to stuttering, or combining frames which leads to blur. So any lower framerates then you actually shot always will be worse.
On the other hand, most if not all 120FPS videos are not shot at 120FPS, but only interpolated.
You can think of it like frame interpolation of TVs, that actually display 100/120/200/240 or whatever Hz from a 25/30FPS input.
And it is at least questionable how much this could improve the result, especially if the video was just shot optimized for 60FPS and not with upsampling in mind.