Not OP, but I'd counter that evolution is doing an imperfect multivariate optimization over a pretty big state-space. Even if it might be easy to change one simple chemical and thus speed up processing, the question is whether the speed-up has other costs.
E.g. the brain is already using a lot of energy, increasing processing speed which probably increases energy usage might not be optimal.
We still don't understand sleep well, it could be that if you process faster more consistently you might need more sleep to organize your thoughts and memories, which is counterproductive if you don't want to be eaten. It's hard enough with our sleep requirements to keep up with nature.
A third point is that evolution only needs to find good enough. We're already the apex predator due to our ability to think. Anything above what we currently have might just not be necessary so has no evolutionary pressure.
What if the only thing it's capable of doing is perceiving the passage of time? If the biological clock is the only measure of perceived time, then it feels like more time is elapsing. If the brain measures and feels time as sand slowly falling into a bucket, what happens when you dump a truckload of sand into the bucket? That doesn't mean that you're necessarily capable of doing more work in that time, but maybe you still feel the additional passage of time.
Nonsense. Where's the thousand years of creative output? If my mind existed for a thousand years I'd be immediately filing patents and writing papers based off all the things I came up with during my eternity of thought. Further, if there was a simple chemical way to accelerate actual brain processing thousands of times, you don't think evolution would've built it into our brains? There is no difference between this and hypnotizing someone to believe they've lived an eternity.