When Jan and I evaluated XMPP in 2009, we found that it was not very mobile friendly. To provide a couple of examples -- (1) The login path required an inordinate number of round trips (I think it was 4+). This slowed down login quite considerably. (2) XMPP is byte verbose and expensive on mobile networks.

We came to the conclusion that XMPP was built for desktop computers connected to strong internet via LAN connections not for mobile networks. We went on to invent our own protocol which was byte efficient and minimized roundtrips.

Now take all of this with a grain of salt. It is now 2018 and things have changed considerably.