I've been playing around with something like this.
Slowing things down has an interesting effect on gameplay. So much so that I think the game was originally intended to be about three times slower than it currently is. By slower, I mean in the time it takes to do things like research, build, travel, colonize planets...
Some things I've noticed are that there is no need to do an early mineral rush, edicts make more sense, some of those techs like decrease colony time or improve survey speed make alot of sense, etc... And it doesn't seem to throw the entire economy out of balance... At least yet.... I'm still playing and only in early game with my latest changes. I haven't even looked at end game stuff yet.. The time based one sounds concerning.