For me a big deal is not just the slow delivery, but the inability to instantly send as well. The presence of a send button has this effect of making me send things that aren't ready. I don't intend Pony to encourage people to edit and edit and edit forever, but I've found in my personal usage that sometimes I write a message, put it in my outbox, and then some number of hours later I'll think of something randomly and edit it a bit. Or I'll change my mind and delete it.