Define slalom? When talking about formula we all probably think of a 100 cm wide board with 70 cm fin and monster sail on $$$ carbon rig.
When it comes to slalom both an iSonic 144 being 85 cm wide able to carry a 58 cm fin and a 60 cm wide board with a 32 cm fin can get going in 20 knots of wind. Pointing and speed can vary a lot within this spectrum. Some people claim that even the "old" Hypersonics were able to go near formula boards in pointing and downwind sailing, and, if the sea is rough, probably with a higher average speed through the water.
Anyway on round island races in my local area the quickest around a course this year was... a kiter...