Some of that is true, but XML and the XMPP protocol are highly compressible.
XMPP uses a long-lived TCP connection between servers, so the continued exchange of <message from="foo@wavesandbox.com/Wave" to="bar@example.com/Wave"> will compress to substantially fewer bytes on the wire.
The XMPP Foundation has done a lot of work on this, and continues to work on scalability issues; particularly between federated servers. Indeed, XMPP has reached a point where it's a fairly mature, secure and speedy technology — clearly making it desirable for companies such as Google to use as a foundation for interesting applications such as Wave.
Ah, didn't realise only IE supported defer!
I never quite understood the HTTP/S detection, as I would have thought it'd be simpler to do "//google-analytics.com/..." (i.e. no explicit protocol). Though again I don't know what standard, if any, defines that behaviour. But if it is/was standard, then you could have a single script block. But of course, as you say, developers who call _gat from elsewhere (such as myself) would have to know when the content had loaded. So, er yeah.. forget all of that!
I guess inconsistent browser is also why they disabled gzip support? Though I would have thought it would be trivial to Vary by User-Agent.
Or they could probably just use the "defer" attribute on the script tag.
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