It's easy to think about why StashingAway is correct here - my brother is a GC who does small/custom jobs and here's the process:
-Bid out 10 different jobs. Only get 1. You're going to pay yourself for your time, though, so all that bidding work is going to end up being part of the price of the job you got one way or another.
-Try to schedule your subs to show up at the right times/in the right order. Sometimes this works, usually it doesn't. Time is money, and nobody can start framing until the foundation is poured.
-If you have big equipment, trailer it to the jobsite and leave it there. If not, go rent it from Sunbelt. Sometimes they'll deliver it, sometimes not. More $$$.
-Try to schedule inspections to work with your subs, try to get friendly inspectors. Sometimes this works out, sometimes not. If you fail for one tiny problem, gotta get that sub back to fix... could be a while, and you'll pay to have them come back out.
If you're building spec houses, different story. You aren't spending your time bidding stuff out or walking around in some homeowner's backyard talking about their new sunroom. You own, or long term rent your big gear. No schlepping back and forth. You dig a foundation every day and keep the excavator busy. You employ the concrete guys, framers, plumbers, painters, etc directly and they only work on your jobs, so you can schedule them the way you want. City inspectors know the project and your guys know the inspectors and things tend to go smoothly. You can get a dozen 4-way inspections done in a few hours in one morning and have your guys standing by to make fixes immediately if needed, not 3 weeks from now.
I mean, it's WAY more efficient. There's just no way around it. Anyone who is even vaguely familiar with construction knows this.
I'm not saying that spec homes are inherently better or anything. But they're cheap vs custom to build for a reason.
-W