I live in an apartment now, but prior to that I lived in a room over top an un-heated garage. The duct work ran the full length of the garage to the register in my room, so while the rest of the house managed 68F, my room struggled to get 62F. Needless to say, it was bad enough that I forced my parents to invest in a space heater, the oil radiator type. 600 and 900 watt coils, and 1500w with both running.
I never used more than the 600w setting in that room, and with that same heater in my apartment, (710 sq. ft.), I managed to run just the space heater down to 40F. Below this it just won't do by itself. But it does make it much more comfortable in a room allowing me to run my 9kW
central system at a lower temperature. (No heat pump here, all electric heat).
I think radiator space heaters are a lot better than the built-in fan type. Slower, for sure, but much higher efficiency.