Framegen is worse the lower your base frame rate is.
The penalty to the speed at which the game runs is much more significant, if you normally run at 40 fps and framegen gives you 60 (30 real) then you have introduced 8 ms of latency just from that. While the same 25% performance cost going from 180 fps to 270 (135 real) adds just 2 ms.
The lower your real frame rate is the harder it will be to interpolate between frames because the changes between frames are much larger, so it will look worse. Also the lower your frame rate the longer any mishaps will remain on screen, making them more apparent.