Before you try anything else, make sure it's not a case of your equipment not working correctly. IOW, first try doing facial mocap WITHOUT the body mocap to see if it's any better (you don't say how you are doing it, whether you are using Live Face or Faceware, but I do know that RL has done work with using Faceware and the PN suit and the capture is fine. I also know some folks who have improved their Live Face capture with various means).
Assuming for a moment it is NOT your facial capture setup, you can always smooth any motion out by sampling the clip, squashing it (to like half the length) flattening the clip and then stretching it back out again. This is a very standard process but you need to be careful you don't lose some quick movements you otherwise want (so you might not want to do the entire clip).
Usually it's the body mocap that has issues (which is why I am suspect about your setup).
Alienware Aurora R12, Win 10, i9-119000KF, 3.5GHz CPU, 128GB RAM, RTX 3090 (24GB), Samsung 960 Pro 4TB M-2 SSD, TB+ Disk space
Mike "ex-genius" Kelley