The time to produce the next block is completely unaffected by the number of unconfirmed transactions.

The time is generally expected to follow a Poisson distribution as explained in answers to a related question.

I’m no statistician so what follows may be nonsense:

A Poisson distribution with a lambda of 6 shown as a probability mass looks like this

So in a thousand hours I might expect to see

- 160 hours where 6 blocks were produced
- 45 hours where 2 blocks were produced

Either that or something odd has happened, like a couple of large mining pools going offline.

It would be interesting to gather some of these sort of stats from the blockchain record but I expect this has already been done somewhere.