Thanks Richy, great fun was had.
The Mig 21 FPS seemed to be very bad near ground detail, low down, making it practically impossible to land.
I managed to land MiG-21 perfectly at the end of the session. FPS was OK.
It seems that two or more MiG-21s on the same airfield will cause performance drop, or MiG21 + Sabre etc.
However, the developers promised that performance issues will be fixed soon.