Quoting David C. of the TES3MP project:
"The physics thread is one of the main bottlenecks in OpenMW, and setting OPENMW_PHYSICS_FPS to 40 or 30 provides a quick and somewhat effective solution to it, with side effects that are not very visible or important to most people who have changed it and reported back."
Indeed, after noticing that physics were a bottleneck in my own setup (using F3) and finding the above post, I tried it myself. The results were absolutely noticeable when I set it to 30. Not a wonder fix by any means, but in some areas where FPS suffered the most (Balmora, Vivec) I saw FPS gains via F3.
Unless you are getting 60+ FPS everywhere, all the time, I highly recommend that you at least give this a shot.
How to set environment variables is beyond the scope of this article, so as akortunov advises: "Just google "setting environment variables in [insert your OS name here]" and you should come up with some good info.
Did you use this setting and notice a difference? No difference? Please let me know!