If time was faster or slower, you wouldn't notice anything. Because time is an expression of causality, and if causality is either faster or slower, it would go for everything, including you, your phone, your watch and everything else.
If time was slower or faster, there would be no way to detect it.

But we do have an inner clock, and that clock slows down as we age, making things feel/seem faster.
Many animals have way more accurate inner clocks than humans, for instance cats can tell the time of day very accurately, and are known to have daily routines on the clock, also disregarding sunlight and owner behavior.
Most animals also have a way more accurate perception of speed of events, and will often seem to react only in the last second, but will very rarely fail. This can be seen with for instance pigeons in traffic. That will only move at the last moment when cars or scooters or bicycles approach.

So whatever number of seconds it takes a person to count Mississippis or run 100 meter, is purely subjective, and in no way a reliable measure of time.

I specifically remember counting chunks of time as long as 15 minutes and not being off by a minute.

I can almost guarantee that memory is not accurate. the human inner watch is simply not that accurate, even with tricks to aid it.
I personally have a somewhat similar memory, of being able to tell the clock very accurately by the position of the sun. But the truth is I probably tricked myself, and when I got it wrong I discarded it, and only remembered the accurate ones. Classic confirmation bias, that we should all be aware of that we are all victims to now and then.