>>2114734 (OP)
The one common resource you manage across all RTS games is your attention economy. The quicker you can formulate a plan for a given section of a game (like your base) or a given situation (a skirmish in progress), the faster you can act on it and move to your next situation.
If you are "slower", you will need to think longer about which build order to use, you will react slower to sudden situations like a raid or gank, and you process less data in the same amount of time someone smarter than you does, leading to more mistakes.
This can be mitigated by just playing the game some more. The more you play, the more situations you have already encountered and you can rely on muscle memory and knowledge to deal with situations instead of thinking about them. The "tunnel vision" happens when you are at max capacity mentally, your brain locks up because it's overloaded with information so it tries to narrow the information stream.
Play more = git gudder. The smarter you are, the less time you need to improve. This is by the way completely transferable to anything in real life.
You do get slower with age. There is nothing to be done about that, it's a biological process.