"I figure that at lower speeds, the air spends more time in the heater
core, and gets hotter, so that makes up for the fact that there is
less hot air. "

Yes, and no. Yes the air gets hotter at slower velocities. And no it
doesn't make up for it. While the air coming out will be significantly
hotter, you still get the most heat by having the max blower speed.
Max blower speed forces more cold air into the heater which extracts
more heat from the heater core.