There are some Macs that do not support hardware accelerated h.264 encoding. For the best possible frame rate try using “Encode Later”. Then bring up the History where you can save the encode the recording as an h.264 or Apple ProRes movie.
If a lightning bolt is visible next to Video then your Mac will use GPU accelerated encoding for the settings selected.
We always recommend doing some test recordings when possible to determine which settings you should use to record.