I wonder too if this is a maybe problem of aircraft.
I have tried the same airport (LFPG Charles de Gaulle) with different planes under the same zome and sometimes the fps drop sometimes not.
It s not a kind of complexity of aircraft or file cache because i reproduce the bug several times.
R5 3600 + 32Gb+ SSD+ 1060 6g
2 screenhots :
LFPG with beechcraft : severe drop of fps from 30 to 16 with gpu 100%
LFPG with A320 : no noticeable drop and gpu at 89%