Motion blur is obviously more intensive than changing colors.
and 5 FPS OH MY GOOOOOOOOOOOOOOOOOOD
And the 1 FPS difference is not a guess.
If it looked decent, they would make it an option. That's why we have graphical OPTIONS. You can just turn it off if your computer can't run it. If your computer can't handle this, you shouldn't attempt to run this game in the first place.