I just bought the game and I was wondering why it was stuck at 30FPS, especially after the high rate refresh screen warning.
That is not a performance issue at all, as the GPU is just basically asleep (and so does the CPU).

I finally noticed that even though I was in VSync ON, the frame limiter set at 60FPS, was doing something :
if I set it to Unlimited, I got my actual screen 120FPS.
Therefore, I digged a bit more and got the following frame rates :
Unlimited => 120
60 => 30
48 => 24
30 => 15
So half the intended target, which I guess is not the intended behavior (neither is in my opinion, that enabling VSync still get the limiter active even if it appears greyed out).
A thing that could be related is that I have 3 screens: one 4K@120Hz VRR where Endzone runs, and two 1080p@60Hz.

I also noticed a side effect of the framerate changes, which is that the keyboard camera move/rotation input us actually linked to the framerate, a 1 second key press will rotate about 1080° at 120FPS, whereas at 15FPS it will rotate not even 180°. But given there is a sensitivity setting, that is not a big issue.