As far as i know, Karma currently is not consistently faster than Mantra (means it is faster for some things, but can be even slower for different things). so far, my superficial experience with Karma made me put it aside for now, as it still seems to be a very unfinished product, unfortunately. i'm still very excited about Karma thou, and maybe Houdini 19 (or 19.5, 20, ...) will bring some significant updates to it's stability, speed, features and general user experience. Up to that time i'm gonna continue living in Mantra world.
I have no solution for your "not being able to render stuff out" issue, but it sounds like Karma to me.
Also, I hope to read some optimistic posts in this thread