Thanks for your suggestions ! The game is written on Pico8 fantasy console. If it is possible to write a game relatively fast it's because it comes with a lot of constraints, such as limited resolution 128x128px, 16 colors, 4 channels sound and 8192 "bytes" for the code. I already have touch this limit multiple times and I can't figure out how to optimize more to free some bytes.
As an alternative to boost pad, you can gain some speed by driving on the half pipe. ;)