Honestly, I wouldn't be surprised if it was intentionally slowed down. They made great pains to say this isn't something that should be rushed. Could be the timegating was a way to soft force that in game.

I will say, to the point of the gameplay part being slow, I've found it kinda isn't.
You can collect the mats, queue up works and building, setup crops and the like. But those bits don't actually take that long once you know what you're doing.

It's the waiting for stuff to be done that takes up the majority of the "work". At which point I've just gone and done something else.

All told, I haven't actually spent as much time on the island itself as I was expecting, beyond the initial startup stuff.