there's a reason they call it speed. It's an accelerator; it takes whatever your mind is aiming for and accelerates you to it. Due to the high amount of raw physical resources required for this, the brain literally can't maintain the upkeep required for prolonged use of it - and starts having to make compromises.

Right away you'll notice that sight and sound begin to become hallucinated, as your brain can't afford to spend the resources needed to update those things when they might as well be automated. Basically, your brain attempts to fill in the blanks as if you were dreaming