I can’t be bothered to read through 967 
posts so this might have been mentioned before but for me performance is now is back before the UK update and is great.
What fixed performance for me was following this video which I had done before but the last patch had reset the change. I must admit though I only usualy fly in rural areas with GA planes.
Let me know if you try this how you get on. 
Microsoft Flight Simulator | Stuttering From Last Update | Fixed!!!
Ah just noticed @BrawnierHook18 has posted earlier, sorry about that.