I am away traveling so I won’t be able to investigate this for a couple days, but one thing that puzzles me about your request is that 2.1 did not work with P3D at all. One thing to try, start the windows task manager and and open the tab where you can monitor cpu usage, leave it open and start a flight in the sim with the client when you notice the slow down switch out and see how much cpu the client was using.
Very little changed in the sim to client interface so i am not sure a reason for your slowdown.