You can change the game speed of SC and/or simply pause in single player and give orders without worrying about reaction time. But I totally understand that it’s not for everybody!

Building times are long, yes. But in this game, more than in any other, it is important to support construction with additional workers. Everything that produces something, be it workers building something or a factory producing a unit or a missile silo creating a rocket can be supported by construction units. So while an experimental unit can have something like 120 minutes as building time, that is for one single unit. So you built that with 30 workers and then it’s only 4 minutes. Provided your economy can sustain the needed resources. But that is what the late game is about: building a strong enough economy so that you can output silly amounts of the big units.