Gearing on a Slash is tricky, it is a pig. The fine balance of speed & heat come into play.
The size of the track & the number of turns dictates ratio a little bit.

The Stock spur is usually good enough, I used to use the 90t spur on tighter indoor tracks with smaller pinions.

Once I shredded the second spur, I switched to the Jato 0.8mod spurs... 56t & 58t with a 32pitch pinion 11, 12, & 13 tooth.

Setup & good driving will make a slower truck faster than the crazy speed demons on the track.