The article makes a few interesting points, but fails to make the distinction between the two modes in which one can assess the slowness of time: in the moment, and looking back.

Doing something dull, repetitive, routine will seem to take a long time in the moment. However, looking back it won't leave the impression of a lot of time having pased, as your brain will compress similar experiences into one.

The article argues for trying to slow down time looking back (which makes sense to me), but some comments in this thread (bringing up things like meditation) are talking about slowness it the moment.