I don´t know if this is a temporarily issue, but since WU4 my sim is unplayable.
I have to turn off Multiplayer (General → Data) so I can play.
I´m not talking about 10 fps drops, I´m talking about unplayable fps, like 1 fps.
Try that if you have the same issue as me and let me know.
World updates attract a lot of visitors. London was exactly the same. Eventually people have their fill then fly elsewhere, so things will eventually improve. Consider disabling MP traffic if it is too impactful.
I played with all online functionality options and the only one that did the trick for me was Multiplayer, yes.
I hope this gets sorted out quick
I had the same issue with low fps and serious stutter, turning multiplayer off redeemed smooth flights again. however this stutter only effects busy airports, like heathrow london EGLL
Same 
Cannot fly in multiplayer…alone yes, Tags only have names, no type/alt most times. Flyers switched to all the servers to try to get a handle of what is going on…no joy.
Stutters and 50% drop in FPS makes the Sim useless.
Thank you, thank you, thank you!! 
I was going to throw out my computer through the window but this saved my day! I flew over Paris having about 1 or 2 fps and changed almost every setting on the graphics tab without result, I had no idea this multiplayer setting had such a dramatic effect. Asobo must add some logic here which should reduce how much traffic the sim receives. If fps drops below a certain value the sim should just turn it down to zero.
Thanks for pointing this out - it seemed that if I flew the FlyByWire A320 or the Aerosoft CRJ I would get 2 FPS and couldn’t even start whenever I picked Dallas (KDFW) - I guess it’s always busy in the Multiplayer world - also the same for KSEA and KJFK. - I have to just turn multiplayer completely off.
Hi all,
Please use this topic here:
https://forums.flightsimulator.com/t/bug-logged-mutli-player-stuttering-after-world-update-bug-report/391922
This topic has been pushed up to the developers.
Thank you!