Just the process of getting the prongs into place then getting them out. Its a common sight to see lifters struggling with their belt, particularly if they are new. It's similar to shoes that have laces or shoes that have velcro or shoes that are slip on. Its not a problem initially but if you have to put your shoes on 10x per day, after a while, you gonna go with the slip ons.
In practice, they are pretty much the same, with double prong being slightly more stable under load than single prong. It's just getting da fukkahs on and off so many times in a training session that will have you wishing for a lever belt.