I find pedaling standing at 90-100 rpm in a lower gear (bigger cogs) tires me much faster than pedaling standing at 60-70 rpm in a higher gear (smaller cogs).
Ditto. I find the granny gear while seated more tiring for steep uphills because I lose any momentum, and end up going so slow I also have to concentrate on balance. After trying it a few weeks I'm finding the granny gear mostly useful for gentle uphills at the end of the ride when my legs are dead, usually the final home stretch which is a long, gentle uphill grade. And it's occasionally useful for lugging 30+ lbs of groceries home on that same long, gentle uphill grade.
For short, steep climbs I'm finding it easier and less tiring to stand in a taller gear and keep up some forward momentum. But my conditioning is iffy so if I haven't reached the crest in 30 seconds I'm gonna have to walk the rest of the way up.
Subject to change as my conditioning improves.