That's pretty, much it. If you start in the parking lot with divers of more or less equal experience and speed, with the tanks set up and the exposure suits on, it's generally about equal when both SM and BM divers gear up and don the tanks in the parking lot.
If the SM diver walks the tanks to the sink, it can be either faster or slower depending on the sink, the distance walked, etc.
And in either case, it's a matter of experience and organization. I can clip on the left tank, connect the inflator and ready the reg, then clip on the right tank and reg and be ready to go in about the same amount of time it would have taken me in back mount, but it did not start out that way. Walking the tanks to the sink is also easier given the lower center of gravity, and you have the option of carrying them separately or together before you gear up and then clipping them on in the water.
If I polan ahead slightly I can set up one tank in the motel parking lot that morning and then walk it down to the sink when we do the pre-dive inspection of the conditions. Then I am only prepping one tank and then carrying only one tank after I gear up. The other goes on in the water, but I'm usually on or ahead of the time curve for the team.
It's also more complicated than just "time":
1) There are places where you can easily get out in a pretty normal fashion in sidemount where a back mount diver is pretty much screwed and will have to remove their backmounted tanks, lift them out and carry or cart them back to the vehicle - where I will by then be all packed and ready to leave. When the water was way down at orange grove we dove it with no issues while many back mount divers just passed on it given the difficulty of getting out.
2) I've never seen Marci crash in sidemount but she's fallen a few times getting in or out of some of the more challenging sinks in backmount.
3) sidemount gives you several options in terms of what, when and wear to gear up to match the terrain, the sink, the boat or the distance to the sink.