I get under 20 fps when taking off, once in flight for about a minute, the frame rates improve. I am flying under 60 fps, but very playable in the 40's.
Is this common ? low/lower fps at takeoff ?

Short answer is "yes".

Long answer is your CPU has to process all the items located on the airfield, so it drags your FPS down. Easy fix is lower your airfield intensity in the Workshop setting and it should raise your FPS during take-off and landing.

Thanks.