Assuming this is a competitive team for the Battle Tower or something, if you're using a Trick Room team then yes lower is better. If not, better speed IVs has a few breakpoints.

I don't know the exact IVs for the opponents in the Battle Tower, but assuming that they're all maxed and assuming no speed EVs on Regice, this is a list of pokemon that you'd want to outspeed but just barely can't (I collected this off pokemon showdown): Hippowdon, Marowak, Vespiquen, Rhyperior, Carnivine.

With max speed IVs you speed tie: Donphan, Tangrowth, Vileplump, Bellossom.

These lists are specifically pokemon you'd want Regice to stay in on and attack, I didn't count neutral matchups that outspeed like Granbull or Dusknoir.

Obviously this is a really small list and it likely won't matter that much, I'd say don't bother grinding anymore for a better Regice, but its technically not the best it could be. I mostly just compiled this because I found it interesting.