Remove potion effects using command blocks
Solution 1:
The command for getting rid of all effects is /effect *player* clear, so if you put a pressure plate or something on the finish line connected to a command block, the block command would be effect @p clear. If however your racers have other effects that you don't want to remove, use effect @p 2 0, which will remove slowness only.
Solution 2:
You can remove slowness from a player using
/effect <player> 2 0