The main problem with HTTP is that today, it’s used in a way that it wasn’t designed to be used. HTTP is very efficient at transferring an individual file. But it wasn’t designed to transfer a large number of small files efficiently, and this is exactly what the protocol is called upon to do with today’s websites.
To overcome this and many other issues, SPDY uses a single SSL-encrypted session between a browser and a client, and then compresses all the request/response overhead. The requests, responses, and data are all put into frames that are multiplexed over the one connection.This makes it possible to send a higher-priority small file without waiting for the transfer of a large file that’s already in progress to terminate.
Compressing the requests is helpful in typical ADSL/cable setups, where uplink speed is limited. For good measure, unnecessary and duplicated headers in requests and responses are done away with. SPDY also includes real server push and a “server hint” feature.
Google claims a 50% reduction in page load time.
Source: Ars Technica.