Try to allways have 2 saved positions: 1 with clear geoscape, and 2. for ex. "before mission", with UFOs, missions, planes in the air etc.
Each first attempt (after starting the game) that I try to play tactical mission the FPS goes rapidly down. Then I have to abort the mission, load the 1 save slot with clear geoscape, then load the 2nd save again and the FPS is OK. But sometimes I have to exit the game and start it again. I tried to change some values in graphic settings, but it has no effect, so I realized to play it on the 1024x768 with highest texture resolution. Maybe this trick with clear geoscape help someone.