> All you ever see is brain activity!

YES. To put it crassly, it's like your brain is The Matrix.

I was driving the other day, thinking about reaction times and had the actual tangible feeling that I was just behind 'reality' because we don't actually see, feel, hear reality we see, hear, feel the brain's interpretation of it, and so it's ever so slightly behind 'actual' (are there variations in processing speed? And how do the fringes of these variations affect one's ability to exist in society?).

We are all living a very recent, rolling memory. Operating system loaded into RAM.

Related to this, I think the phenomenon of "being in the zone" is where the separation between reality and what we perceive of reality seems to get extra narrow (to the point of thinking that there isn't a separation between the two). In times like this, we tend to do things "without thinking" (such as in the case of athletes), but I suspect that we are either processing things (a background level of "thinking") so quickly that our minds can't/don't have time to explicitly form thoughts about reality and respond to these thoughts, so eventually they give up on doing these things during the duration of much of the remaining time in the zone, or so processing things so correctly (matching reality relatively better than in other times/contexts) that it's notable.

Some professionals take issue with this framing, but it's usually referred to in jargon-permitted settings as "controlled hallucinations".

Why do they take issue with it? It seems obviously true?

We've clearly got inputs, outputs, and when working correctly, some signal processing going on in the middle.

For one, the hubris of obviously-uninformed statements such as this one.

But more like: the inputs, processing, and outputs have much more complicated relationships than this causal chain that you have described. Modern neuroscience and research into NCC have shown that the brain, obviously, is not so simple.

Maybe they're the sort of professional who also get tax exemptions on religious grounds

A foolish supposition that draws a pre-supposed (and patently, absurdly false) binary across what people are allowed to believe.

I know there’s little tricks you can play with your visuals/perception of time by looking at clocks and away quickly.

yeah, those color illusions really demonstrate this. like when you stare at a point and the picture changes, the picture looks fully colored but in reality its turned black and white and your brain just hasnt realized it.