I did not own the mig-21 prior to the free period so I cannot compare to the previous performance, however it does seem that the frame drops progressively overtime, almost as if clutter from previous sweeps of the radar scopes builds up. If I switch the scope to standby and then back to operational, the frame rate returns to the lower level, however if I allow the radar to remain in standby mode for several seconds, when the radar is returned to operational mode, the frame rate will be higher and begin to decay again as described earlier. Perhaps this behaviour may hint at the cause.
*Edit* This observation was made in multiplayer on the Persian Gulf map. My single player performance does not show obvious frame rate degradation.
*Edit 2* Rejoined after reseting Fxo and Shader folders. Now seeing stable frames.
*Edit 3* Upon checking frame rates, frames with the radar off go from 100fps average, to jumping to between 95 to 45fps constantly. Same pattern to the degradation of FPS as mentioned above. Can get the minimum framerate back to 59fps by disabling the mirror.