This has been a problem for a year now and there are quite a lot of topics on this already.
I fixed it once myself by toggling the AI traffic and in another instance by toggling nameplates during flight.
More topics with the same issue:
https://forums.flightsimulator.com/t/will-you-fix-performance-degradation-on-long-haul-flights-with-su6/460621?u=cptevillian
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