They've recently revamped it so you can adjust various levels (skill increase, cap, multipliers, etc) and make it possible to play with fewer. I've actually run a couple of single-player games, completely vanilla. Fun in a different sort of way.
It has a base setting for something like "1-3 players" that's pretty quick, and there are some advanced settings with which you can make things "cheaper" -- both in time and in resources -- via some coefficients.
I also "cheat" at the start by using vanilla commands to research and level up in all the disciplines -- makes the game more about finding and efficiently utilizing resources without also having to scavenge random stuff to "research". Since there's such a breadth of everything, it reduces the grind ("specializations" are annoying if you're forced to be a jack of all trades) without making it too easy.
It still feels like a real accomplishment to build a large building -- both architecturally and via thinking about how each block traces back to the resources pulled out of the ground -- without requiring huge amounts of time in the game. I'm fairly proud of this one [1], and more so of the industry and infra that supported it.
Plus, Eco is just gorgeous, especially when in single-player with low ecological impact there are so many animals hanging around all the time (peep the alligator in that shot).
