Pure is right, the poly tools don't work on subd surfaces.
Sounds like you didn't convert to polys, just moved into the 'poly proxy' mode of the subd surface (which gives you a poly cage linked to the subd). You can convert to polys at level 0, that will remove any higher level edits from the subd surface, but will yield a clean lowpoly mesh. (edit -> convert -> subd to polys) The true poly mesh will be alot faster to edit than the poly proxy. You can then reconvert back to subd when you are done editing.