A rewrite of the game code significantly relieves the processor.
Microsoft Flight Simulator is one of the best games I've played, but it's also really resource hungry. Update 5, which will be released on July 27, seems to come with a real increase in performance.
In a stream on Twitch, Sebastian Wloch from Asobo Studios says that they rewrote part of the game's code and thus managed to lower how much CPU power it needs. The whole thing is demonstrated with a game round over New York with update 4 installed, where Wloch's fps is at 30-40, with 100% CPU usage. When he then switches to update 5, his fps increases significantly to 54-60, with a CPU usage of 75%.
It should also be added that this performance increase is not due to the planned implementation of DirectX 12, which will come at a later time. Update 5 will be released on July 27, which also happens to be the date when the game comes to Xbox Series X / S.