My group plays using Roll20 in 3 hour sessions, up they are very flakey and some show up half and hour after we were supposed to begin. This means it takes forever for us to get started and we are crawling through our campaign at a Hutts-pace. We still have fun, though I wish they were more dedicated.
It's worth spending the time to politely express your displeasure with the constant tardiness. They're stealing time from you and the other, more dedicated players.