Oh dear!
I recently installed FSX for the first time, and after setting bufferpools=0 consistently achieved 30fps locked to half refresh rate.
Then the Acceleration pack arrived so I installed it straight away. Guess what? My fps halved!
I double checked I didn't have DX10 enabled, then I ran fs9 on the same pc which still sat at a solid 30fps.
So I uninstalled Acceleration, and the FSX fps stayed at half what it had been prior to installation.
I then restored the system to pre-Acceleration status, and lo and behold - solid 30fps once more.
I really want to use Acceleration. Any ideas anyone?