ANSWER: The problem is the muffler/Exhaust port on the bottom of the gun handle. On the bottom of the handle is a hex head bolt with a 1/8" hole in the center. That's the breather/filter - assembly # 22.
Remove the part and your speed should be normal. I left mine out, but you can push a nail through the small screen. As foam and grease collect on the screen, the exhaust leaving is slowed down causing the shift force to work against itself. Many factory people will claim you have to have that to prevent foam from entering the gun handle.
I say - If you have that much foam blowing in the air, you have far bigger issues to address.