If all the people are willingly ready to hear the sermon or tafseer, then it is allowable to deliver a speech before witr provided the speech is of short duration (four or five minutes) not so long that people start feeling bound and boring. However, it is better to do it after performing witr so that those waiting for witr may not feel pressurized. Moreover, some among them may be ill or having other needs, so it is not good to get them bound for a speech:
قال رسول الله صلى الله عليه وسلم : إذا صلى أحدكم للناس فليخفف فإن فيهم السقيم والضعيف والكبير (مشكاة: 101)
According to Hadith, when there is a ruling to shorten in regard to Imamah taking care of the people, then how can it be allowed to lengthen the procedure with things other than salah.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband