We're talking about perfecting assassinate, not being able to assinate quickly, which is what you get once you do perfect it.
And few if any people sub-hero are raiding anyway, and even fewer without a big group. The raid goes pretty quick. Once you have assassination perfected, yeah, you can get someone in the process, and I think I did once or twice. But until then, no, you won't have the time.
The reason it's so much harder as a rager is that people can get away from you in ways you can't follow, and go places you can't. Non-rager assassins can recall when their prey does, or teleport to find people, or use invis to hide from people who can see hidden, or camo (rangers in wilderness), and things like that. It's the primary reason I was only the second rager to perfect it. I'd welcome others doing it or trying, and if they can do it substantially sooner than I did (without obviously violating all sorts of rager rules) I'd be surprised.