If you load a save game mid mission, you may experience a severe framerate drop. The only known workaround for this is to completely restart the mission, likely from the main menu (not from the F10 menu).
Note that should you restart the mission mid-game as you may experience a lowering of your unit caps.
Both situations have been presented to Relic for Resolution.
Note: These were not duplicatable in testing for the 1.1 patch, this may mean that they were fixed by something else, or the recreation steps were not available.