NNewcomb wrote on Dec 13^th, 2011 at 3:13am:

I was just flying the P51 included in FSX and I noticed that it is very slow and very hard to maneuver. I can only get a top speed of about 100 mph even though the real P-51 has a top speed of 436mph. I also felt as if I was always just above a stall. The aircraft is hard to turn and hard to fly straight. Does anyone else know about these issues and do you know how to fix them?

The solution is to wait a few months and buy the A2A Mustang with Accusim.