I have a spa with a 220V element running on 120V 15A because
it is wired for it and 220V is unavailable at the location. I
realize it runs at 1000W. My question is: would a true 1000W
heater heat the water back to temperature quicker (the
4000W element seems sluggish: slow to heat up, then
slow to cool down).

Dr_Spa

no

chemicalfree

Basic physics it's a functon of input.