Before Einstein there was a theory that moving objects just got shorter and their processes took longer to complete. Becsuse of this, we still use the words length contraction and time dilation. But they are horrible names, and there is a problem with the whole idea.
If you theorized that moving objects slow down and get shorter. Then the moving objects wouldn't notice they are moving. They would think you are moving. They would think you are shorter and you are running slower.
So they'd think you compute everything with defective slow running clocks (that then don't measure time accurately) and using defective shortened rulers (that then don't measure distances accurately).
So there is a huge problem in that you theory thinks there is a magic frame where clocks at rest measure time accurately. And where rulers at rest measure distances accurately. But there is no experimental procedure to find out which frame is the magic frame.
What we do know from experiment is that two clocks tick different based on how they move. And two rulers measure differently based on how they move.
And our thekries should be based on what we measure and not on undetectable magic frames. After all, in the magic frame time is measured by clocks at rest in that frame and distance is measured by rulers at rest in that frame.
So instead of magic, we could accept that clocks and rulers measure something and that what they measure depends on how they move. And now we don't need to treat any one frame as special as long as we make a theory that predicts accurately and correctly what real clocks and rulers do.
And that theory is relativity. It says there is a thing called a metric. And clocks and rulers measure the metric along the 4d path in spacetime that the clock or ruler takes. One metric. Lots of possible paths. So lots of possible clock and ruler readings. And if we pick the metric right the predictions will agree with what clocks and rulers read.
So we don't measure time. Never have. It's just that clocks that move slowly relative to each other tick at very similar rates. So we conflated a measurement of the metric along those really similar curves with "time".
Now to finally get back to your talk about brains and physical processes. Processes don't ever happen at a certain rate in time. They always happen at a certain rate of the metric as measured along a path. So if you fix a lath, and measure the metric along that path then you get something like a time, and that's what tells everything (clocks, brains, radioactive decay, any process) how often tondo something.