Resistance training: rep duration less important than volume and sets per week

I have wondered what the benefit of slower reps is, but according to this video analysis of research, more important are several factors, including total volume, and number of sets per week. However, the duration should be long enough to be done in a safe manner. I assume that the rep duration can be kept consistent so that progression will be measured by increased weight in sets of similar reps. Again, I think a big factor is knowing the maximum limit to reach over 4 – 9 reps per set and trying to reach it so there the all-important ‘overload’ is reached.