Time neither can "slow down" or "speed up", unless you yourself somehow, impossibly, find yourself "outside of time".

Because if time sped up, your brain would also be going at this sped up speed so from your perspective nothing will have changed!

For all we know Time could be fluctuating all the time, but since we're riding it and a part of that time, nothing seems to change.

What might change is your perception of how fast that constant time is going, from being really bored or from having done so much stuff that time just flies by, you know?