What is the opposite of "peaked"?

Solution 1:

A more frequently used and colloquial expression is bottom out, which means "to reach a point where a decline is halted or reversed" (MW), or "to have reached the lowest point in a continuously changing situation and to be about to improve" (Cambridge).

My productivity bottomed out at the university.

As @Mari-Lou A has suggested in the comments, a related expression, hit rock bottom, could also work. It means "to reach the lowest possible level" (Cambridge).

My productivity hit rock bottom at the university.

This variant doesn't seem to imply the possibility of a subsequent improvement implied by bottom out.

Solution 2:

It's not a single word, but I'd say to reach a nadir

See meaning 2 of this definition (and the example sentence: Nantucket reached its nadir in the post-Civil War period. The whaling industry had become moribund, ...)