How long is the time frame for you from Prep-missions to getting paid out, now that you have it down?
An organized pair can do the scouting and setups in 35-45 mins depending on which objectives/luck/interference, and doing them by yourself will take 45mins or so once you get it down and learn to recognize which objectives you have.
For scouting, you only need to bother scouting for drugs/cash outside the compound if you are going in solo or with 3 or 4 people. The stuff in the compound isn't accessible solo, so if you want to fill your bags solo (which you do), then you will have to scout a little extra during the prep mission. You don't need to waste time scouting 3 duffel bags worth of stuff if since you can only carry 1 duffel bag worth, so there is a lot of time to be saved during the scouting. If you are going in with just 2 people and sometimes even with 3 people, then you can fill up inside the compound.
The most important thing to scout is the tunnel. Don't bother trying to swim as it would take forever, just steal a boat from one of the docks and drive to it. That will unlock the mission to steal the cutting torch, which is much faster than the mission to get the explosives,
For scouting the RNG stuff, bolt cutters aren't necessary if you get the cutting torch so don't bother looking for them. Grappling hook isn't necessary at all but it works if you are going in on land and it's in a convenient place, but it's not worth hunting for and you are generally better off taking the tunnel entrance. Guard outfits are nice when you are learning how to do the heist as they give you some leeway for getting spotted, but once you know what you are doing they aren't worth the trouble, and they are kinda bugged anyways and you will just randomly get spotted even while wearing them and doing nothing wrong.
Doing the setup missions fast, especially solo, needs the Sparrow purchased. You can also fast travel the submarine, which at first is expensive ($10k) but after you do the heist the first time is just $2k, so being smart with fast traveling near objectives and then putting the sub back into storage and call it back again after you are done with the objective can save a bunch of time.
The Longfin is the easiest/fastest vehicle to steal, especially if you own an MOC cab or Phantom Wedge. It's also relatively flexible as far as going wherever you want to start the heist and is generally the preferred method of entry, whether going for a quick run or just a casual run. The submarine is also a decent option but the setup mission for it is more annoying.
For the gun stealing mission, if you get the one that tells you to go to the Merryweather dock, you can fast travel the submarine right off the pier of and then use the submarine periscope to blow up the helicopter and fail the mission so you can restart it and get a different mission. Having to chase the helicopter is much slower than stealing the guns from an office building. The one with the AP pistol is the preferred set usually, but the "Conspirator" set I think it's called is pretty good too.
Stealing the thumbprint scanner or glass cutter are both slow-ish but with 2 people you can speed it up a lot as they have limited spawn points, so send one person to the first objective while the other stays near the potential destinations with a helicopter.
The disruption missions are only worth doing your first time or two, as they will make it punish you less if you make a mistake while you are learning, but even then I'd recommend just killing yourself and using a quick restart. Once you figure out how everything works and a few of the tricks, they aren't worth the time.
The heist itself takes 10-15mins once you know what to do and it goes smoothly. If you are taking an overly cautious approach or you screw it up or get shafted by AI/stealth inconsistency and need to restart, then 20-25mins. One big time saver after escaping the compound is to jump off the cliff at the western point of the island still inside the fenced in area and swim past the skeleton as it is just a short swim to finish the mission.
Even when you don't have it down and are taking longer, around $1.3mil for an hour and a half worth of play is about as good as it gets in this game. I suppose idling in your office while your drug/gun/nightclub make product and then coming back and selling them every few hours is less "work" but it's also far more boring.