We are drifting into unrealistic theories again.
First you compare experienced backmounters to inexperienced sidemounters, then the other way round and back again.
Why should the BM diver be faster? It is obvious he can't be!
For a single dive the backmounter takes minimally more time to unpack and set up his equipment, but is still slower most of the time.
The Sidemount diver just prepares his tank(s) and lead, everything else is dropped into his box mostly unchecked.
Mind you, most sidemount divers aren't experienced enough to do that elegantly, but even most beginners are fast at least, they just forget important things often and have to start over several times.
When gearing up the backmounter loads the jacket onto his shoulders, afterwards everything becomes slow motion action on a moving boat.
The Sidemounter will put on exposure protection, throw his equipment on one shoulder, take the tank in the other hand and finish setup while waiting in line.
With two tanks he will probably close his belt first, but can simply walk into the water then with a tank carried in each hand.
Clip on tanks, sling hoses, decend, putting on mask and fins before is optional - imitating only that part alone requires significant skill in a backmount config, beginners skill sidemount.
With good preparation and the tanks waiting in the water you can do everything in less than a minute while walking across the boat without slowing.
How could any backmounter match that?
Even the most skilled have to slow down a little bit as soon as loading the equipment on their shoulders to not end up on the deck with some important bone broken.
The sidemount diver is either about as slow (but not slower) fully geared up or carries at least one tank by the valve.
He can lean on it for support while standing, is mostly unencumbered, if he can comfortably lift the weight of the tank (30lbs max for single tank).
Fully equipped and set up the sidemounter is much more mobile than a backmounter carrying the same amount of weight and much more secure on his feet.
I have seen people jump over obstacles in sidemount and have more then ones seen someone drop gracefully to the ground in situations you close your eyes looking (where almost any backmounter would be on his way to a hospital afterwards).
We have a few sites here where you have to cross busy streets between car and water, I have seen sidemount divers just start to run and backmount divers try to and just stop after two or (rarely) five steps, resigned to their fate.
I really speak from a viewpoint of moderate experience here.
I was always one of the last to get into the water when backmounting and still often get distracted and talk with someone too long and too often.
On a boat I was horrible - the worst
So when I realized after two or three dives sidemount could compensate for that I only started again to try too keep up with the group, before I had already given up trying.
To me it is not a race, it is a bad personal habit I have to get rid off.
Today I take about 15-20 minutes from the car to automatic activation of the dive computer at 1.5m - if I have to.
I 'always' decend last after entering the water first, most of the time only partially set up for practice and reach target depth first or with the group.
On the boat I feel 'at home' now, I just love it - always liked sea-travel, especially in rough seas, but trying to walk with a backmount jacket, I was only a liability.
I more or less 'run' on boats with my complete gear attached to me, often go back for others to get forgotten computers, lights and cameras.
Went up small stairs to the sundeck (yes, with direction of a crew-member to do it that way, I do not leave designated areas on boats on purpose without being told) to have cameras and replacement batteries brought to me nobody of the crew could identify from description.
I would not do that backmount, wouldn't even offer to try.
To me this has always been a discussion of a diving community that developed procedures for recreational boat diving for more than 50 years and really knows about sidemount for about 5.