Because the brain is the organ trying to measure itself, subjective impressions of the speed of thinking can be very wrong. For example if the sense of time is conveyed by the length of time something is kept in short-term memory then impairing short-term memory can make it feel like your thoughts are moving faster when they are really having more difficulty keeping track of previous items in the time stream.

Time sense is notoriously unreliable. Time yourself counting, trying judge ten seconds when you're excited, and again when you're bored, and again a couple of minutes after you've woken up from sleep. You'll be surprised at how differently time moves, yet in each of those it subjectively feels like it moves the same.

Watch a wheel spinning, and turn the light illuminating the scene down -- the wheel will seem to speed up.

Listen to a song while it plays loudly, then turn the volume way down -- the song will seem to speed up.

Time sense in the brain is very unreliable, and often doesn't mean what you think it does.