When you make any FMC change inflight, the simulation makes calculations realtime until you hit EXEC, but problem is your windows defender starts inspecting the script running killing the CPU, all the simm goes to 1fps at best being unusable.
Please go to your windows defender configuration and set a -folder- (not file) exception for the entire /X-plane/ root folder. This will cure the problem entirely. If you use another security platform different than windows defender, set the exception there.