They are taught neither to go “too fast” nor “too slow,” but those are rather subjective qualifiers, aren’t they?
Granted, it can be so fast that it borders on, or becomes, irreverent; but if the priest went too slow the Mass could run for hours, especially with the longer prayers of the TLM. In days of old that may have been normal, but for most people today an hour is a feat of perseverance. Not to mention a number of people travel for hours just to get to a TLM.
Another factor is, they know the Latin and the prayers very well (though that was not always true before the institution of seminaries in the 16th c.), so it is easy for them to say and comprehend it faster than for the average layman.
There was a Saint, I think St. Alphonsus, who actually gave a number of minutes that the Mass should not exceed, in his pastoral opinion, but at the moment I can’t find the quote, nor do I remember exactly what he said.