I guess you meant using assasinate slows the dps of a rogue? not that rogues are slow melee :-p
But i must agree with this, assasinate is an awesome ability, but with 14 starting int I am able to assasate reliably in epic hard and low save enemies (casters/ranged) in epic elite. Assasinate remains a rather unreliable skill imo, there's alot that can go wrong, putting you on a 12(15?) second timer.
Things that make me fail assasinate are:
- fort saves in epic elite when no low fort mobs are available
- the physical hit not connecting due to lag, bad positioning or strange mob movement
- hitting assasinate right before/after i got hit by a stray arrow/spell that causes me to unstealth
- assasinating a stunned mob that was going to die pretty much instantly anyway, often hurting ki-regeneration of my partymember and lowering my personal dps by not hitting anything nor doing a useful skill for a few seconds
- stealth jumping in at the start of the fight sometimes refrains me from activating rogue PL active, lowering my dps afterwards, it also forces me to use action boosts in the fight instead 1 second before the fight
I'm convinced that in some fights not using assasinate would grant better results, although i always do use it because it is fun. Then again, 100% speccing for this ability seems somewhat irrational to me, although it should certainly be fun.