What you're describing is called "kiting", and is already being done by regular players (not to mention the pros). It is only possible for some combinations of units, for example Marines with stimpack vs regular zerglings ("slowlings"), or Viking vs Void Ray because one unit needs to be fast enough to be able to shoot + move out of range before being shot.

Here's the type of micro pro players can do: https://www.youtube.com/watch?v=xrAlhk98WxE±

They do say that they will limit the number of actions taken by the AI to focus on building a smart and strategic AI instead of learning gimmicky micro moves, but if one thing is sure, it's that even the best micro cannot win a StarCraft game if the macro is not on point, and bad choices are made (good luck micro-ing siege tanks against a swarm host!)

Rushing is a bit of a rock paper scissors situation. You can beat a fast rush by doing a slightly slower rush that focuses more on economy, and winning via defenders advantage.

Rush is a bit risky, it has to be invested in before enough scouting & will go up against well micro'd workers

Meta is still pretty fast, here's a SC1 AI finals: https://www.youtube.com/watch?v=LjSXj4cb_Yo

I thought units attacked instantneously, there was end-lag but no startup?