Everything.
The game must handle a lot of stuff for every unit. The more units you add the more stuff the game must handle. At some point your computer just isn't fast enough to do everything in real time, so it starts lagging and uses the bonus time to finish it's workload.
I don't know how many units SC2 can handle safely. It also depends on what units (for example battlecruisers have a very detailed model so they need more cpu power to maintain than a Zealot would). You gotta try that out.
Everything.
The game must handle a lot of stuff for every unit. The more units you add the more stuff the game must handle. At some point your computer just isn't fast enough to do everything in real time, so it starts lagging and uses the bonus time to finish it's workload.
I don't know how many units SC2 can handle safely. It also depends on what units (for example battlecruisers have a very detailed model so they need more cpu power to maintain than a Zealot would). You gotta try that out.
Not really. Over 1000 units is overkill for Starcraft.