Bases take less in-game time to build if you use slower game speed settings.

I've attached a save file which demonstrates the issue. It is building a datacenter with no modifiers to build speed, and with plenty of money to spend. Datacenters have a build time of 1 day, so this one should finish in exactly 24 hours.

Example.zip

If you run on speed 4 till 20:00, then 3 till 23:56, then 2 till finish: Base finishes after exactly 24 hours as expected.
Speed 3 till 23:44, then 2 till finish: Base finishes at 23:45, which is 15 min early.
Speed 2 all the way: Base finishes at 22:37, 83 min early.

I didn't try speed 1 since that'd take forever to run, but since build times only make progress 1/min, It'd probably finish at the same time as speed 2.

I'd expect some small variations in build times from rounding differences and stuff, but saving 83 min on a 24 hour project is a 5.8% difference, which is pretty significant.