Current cvs slime (and, in fact, the half-month-old version too) often neglects slime-restart-inferior-lisp until i call slime-interrupt, by pressing C-c C-c
I am not sure what it correlates with, but this delay, when it happens, is indefinitely long, as it seems...
Unfortunately i do not have much more information at this point...
regards, Samium Gromoff