I know what you mean, it is because for the reprojection to work you actually need more fps beforehand, so if you got lets say 30fps without reprojection and you turn it on, you will be working with probably 25 fps or less, so you will not reach the needed 30fps for it to work.
To find the best setting, enable in the developer OpenXR tool the “frame time overlay”, blue is good, red is less than 30fps. Then while you are in game, set most settings to low, and once you get a stable blue box for most of the time, start increasing settings and effects.
You will be surprised how some settings seem to not hurt the fps at all, even at high settings. Don’t be afraid to set AA and Texture supersampling higher, this might not be the reason you are losing frames. But the biggest impact is TAA from my experience, sadly also the most significant.