It was never really laggy for me when I played it and I played it for nearly 30 hours from what the Nintendo Channel says. I got my moneys worth, I have still never finished the campaign and probably never will because I accidently deleted my save a few weeks ago.
Same, I am looking forward to the Grinder too. Though I feel I might be more dissapointed with it then I was with The Conduit because I'll be more comparing it to Left 4 Dead which I play ALOT of.