Yeah, at best, you want to take 10 minutes if you want the most out of it.
I got about 150k in 90 minutes last night just by doing missions. Weren't trying to take time.. just trying to be precise with killing everyone that showed up as enemies.
Waiting for an updated mission payout list, with base payouts, just to see which mission is the best to "grind" if it was ever needed....
Edit: No.. doing missions faster and paying out more would only encourage grinding more, which is what Rockstar is trying to avoid. If so, everyone would just grind Artificial Scarcity since you can set a team up properly to take out all 7 trucks in about 10 seconds...