If "Effektiviteten" means "the efficiency", how is "effectiveness" in Swedish?
"Effektivitet" means both "efficiency" and "effectiveness", how well something works and whether it works.
That's correct. It's something Swedes get wrong very frequently when translating into e.g. English. :)
The exact phrase "the efficiency is going to" yields over a third of a million results on Google - I think that's quite a bit more common than "rarely", though I agree treating it as a mass noun is more common.