Well, by contributing with faster projectiles you mean them getting a hit in a situation when mobs die almost instantly. Either they "contribute" or not in this situation is irrelevant to the result. In the end it doesn't speed up clear by any meaningful amount( maybe last mob dies few milliseconds faster). Nor does it increase clearing speed in situation when enemies are tough and getting through to your wall, but rather take spot of perks that could increase dmg in some way to actually help.