i would suggest keeping at a 9 gap around 15-18. point is that there are EASY kills in teh den. the archers have low defense and can be easily killed on 9 gap as compared to farming at 9 gap on 20+ mobs. to answer ur quesitons, yes it would be slower later on.
if u really wanna be farmed, i suggest keep farming in the den. don't give in to the temptation of farming later on. u will repent and wish u farmed more in the den when ur sp runs out.

btw this is one of the official normal rate servers right? (i.e. iSRO, cSRO, etc.)