Screen recording is a pretty intensive task so sometimes you may not get the frame rates that you are after.
In general to get the best frame rate possible you’ll want to select either h.264 or Encode Later.
On Macs that support h.264 hardware acceleration selecting h.264 will result in very smooth recordings and very low CPU use. A lightning bolt next to Video indicates that the current settings will be hardware accelerated. Recording directly to h.264 is not recommended when recording a game because we want to leave the GPU resources available for the game.
If your Mac doesn’t support h.264 hardware acceleration you’ll most likely get the best results by selecting “Encode Later”. By selecting Encode Later Iris will record the video to a proprietary format that uses very little GPU or CPU resources. Encode Later is especially recommended when recording game footage. This frees up your GPU so your game is affected very little. Once you are finished recording, bring up the History and export the recording to the desired format.