The biggest factor during the conversion is the size of the cargo door and width of the aircraft. There is a certain size the door needed in order for the pallets or containers to fit. Also the height of the aircraft is also a consideration since you would need a low loader to get containers into the aircraft and then you have the consideration of can you park this loader next to the aircraft. You could have a low loader adapted to do this job but then you have the added headache of how to off load an aircraft should it divert.
As for loading times it is more work to load individual packages but by fitting a roller system to the aircraft and using a cargo belt plus a few more loaders loading time can be relatively quick. This can further be reduced if you can load the aircraft from the back and the front at the same time. Don’t forget the the turbo prop aircraft tend to be smaller so they cannot accommodate as much freight as the bigger jets.