Club speed is consistent throughout the swing with the only exception being the WGT Beginner clubs.
If you are experiencing the meter slowing down or speeding up during the swing that usually indicates a frame rate issue. In other words the flash window itself is having trouble maintaining the required play speed throughout. You might be experiencing high CPU usage and if that's the case I'd recommend turning off a few of the extra options like reflections and turning down the graphics quality to medium or low. Lowering the graphics is a big help for a lot of pc's (mine included) and should help it render faster and perform at a more "even" speed throughout.