How Speedy is SPDY?

SPDY is increasingly being used as an enhancement to HTTP/1.1. To understand its impact on performance, we conduct a systematic study of Web page load time (PLT) under SPDY and compare it to HTTP. To identify the factors that affect PLT, we proceed from simple, synthetic pages to complete page loads based on the top 200 Alexa sites. We find that SPDY provides a significant improvement over HTTP when we ignore dependencies in the page load process and the effects of browser computation. Most SPDY benefits stem from the use of a single TCP connection, but the same feature is also detrimental under high packet loss. Unfortunately, the benefits can be easily overwhelmed by dependencies and computation, reducing the improvements with SPDY to 7% for our lower bandwidth and higher RTT scenarios. We also find that request prioritization is of little help, while server push has good potential; we present a push policy based on dependencies that gives comparable performance to mod_spdy while sending much less data.