I attended too many sub-20-minute masses as a kid.
A Low Mass during the week with no singing or homily, and the usual small attendance usually did take about 20 minutes. I served for several years at these Masses, but I never jumped to the conclusion that the priests did not understand their Latin.
Especially after the several years of Latin they took in the seminary, on top of the usual 2-4 years in high school (since many priests had gone to Catholic high schools in those days).
I only had two years of Latin, but I understand the texts of the Tridentine Mass very well. And that's not bragging, because in general I'm terrible in languages.
So I would think that a priest who attended the Tridentine Mass (with his missal) from his boyhood days, and then was trained throughout seminary in this same Mass, and then offered it every day of his priestly life, would probably understand what he was saying.
If he didn't , I would jump to the conclusion that he was just plain stupid. But I can't imagine how he would get through college and seminary if that were the case, especially with the rigorous training in those days.
So maybe this is just your own opinion?