Exactly changing from Ultra to Low made no difference because the CPU constantly under strain.
I changed everything to ULTRA now increased RS to 180 and changed LOD to 185 now I am GPU limited which is a far more fluid experience.
40 FPS constant at Heathrow, NYC and see up to 50 in the air.
Previous to this change I was tanking at 20FPS at EGKK for example.
Whichever works best for the individual and this works best for me.