google-chromewebblocking
blocking time in google chrome
What does blocking time mean in Google Chrome developer tools and why is it so big?
Solution
"Blocking" is the time the request spent waiting for an already established connection to become available (i.e. we're reusing a connection that may be used to transfer another resource at the time the request was started).
via https://groups.google.com/forum/?fromgroups=#!topic/google-chrome-developer-tools/8jpALcJqkUM
There are a couple of potential solutions to this, split requests out into different sub-domains for other assets. Or look into SPDY