Because in other levels you have more frames left for get the framerule you want and it's not necessary. And if you miss the fast accel you can lose time.

Commentary edited xD

The fast accel in 8-1 is because with the FPG in the end is sufficient for save one more framerule, without the fast accel isn't possible. The slowing down in 8-3 is a setup for the FPG, the time can't be xx3 in the end because the fireworks slow you down and too the setup is used for manipulate subpixels (i think xD)

Nice!