I noticed the slowdown too on Tempest. I used to get 100% full speed. I'm using a P4, 3.0 GHz, Win XP 32-bit.
I don't think it's a bug... it's just that our PCs are old and the game's accuracy is improving, which requires more power and our PCs can't keep up. My PC can still run well over 90% of the games at full speed.