SPDY is an alternative protocol to HTTP that is specifically designed to reduce page load times. I heard that Google Chrome, Firefox, and Internet Explorer 11 support it. Google uses it to load pages. Will Stack Exchange sites start supporting it? More info: http://isspdyenabled.com/ (should give YES result on Chrome, Firefox, and Internet Explorer 11 (in Windows 8.1, not Windows 7))
Nope, not in the foreseeable future.
Why? Almost all of the gains from SPDY come from many requests on a single connection - it's designed around removing duplication and advances in compression over HTTP by not doing extra work, mainly:
All of this is good stuff, it's awesome...if you have multiple requests. However, you don't send multiple requests to us on a page load, we do our best to make sure of that. You make a single request to us on almost every page, then additional requests to our CDN, but not to us. Take a look at this page, what do you get from our data center? The HTML for the page then in the background, later a token that we use to track view counts on questions (but you don't see or feel the load time on this).
If HAProxy implements SPDY support/batching later (not pass through) we'll of course take a look, but as it stands now the protocol itself doesn't really gain us much except another layer to debug and very little to no more performance for our users. Given that, it's just not a win.
Also, practically speaking SPDY requires SSL (technically it's not required, I'm aware), which we haven't deployed yet. We're working with the HAProxy author on a bug that causes our load balancer to overload and stop responding suddenly - let's just go ahead and agree it's best we not deploy that until we find the cause. After that's solved we'll have more to talk about with respect to what's going default SSL - the latency penalty is pretty severe, especially for our non-US users.
When you're as big as google and you're not using a CDN then SPDY makes a lot more sense, not so much for us...at least not yet.