It's slow if you're in a large group, though this is getting better. It's slow if your internet connection sucks. This should also improve as the system matures.

That being said, it's been very reliable lately and offers nearly everything that I need in a messaging app.

Session works, too, though I've never run a group through it. There's plenty of others. Try them all. It's better to spread your messaging out across a bunch of different forms, anyway.