Imho there is no reason to go so slow, and the train risks a soon death if it isn't time-competitive with the minibuses, even on the Izmail - Odesa part.
But the "ustanovlenie storosti" (2013) does show some parts with limits of 60-70 km/h south of Artsyz, while Odesa - Artsyz has more parts where 100 km/h is allowed. This info doesn't account for "temporary limitations" though.
The 902 train Izmail - Kyiv went extremely slow on the Кантемир - Б.Дністровський part, down to 15-20 km/h on a line that is straight as an arrow and theoretically could be a 160 km/h candidate in case of a line upgrade.
Train 902 had to cross with train 686 at Б.Дністровський but it should have been able to leave Artsyz later and make to Б.Дністровський at the same time for the crossing if it had run at the 70-100 km/h allowed-for speed.