If nginx is waiting it is waiting for php to return something or until it times out, in that sense php is blocking nginx, you need to study the logfiles, monitor with process explorer and test with "curl -i" to see whats happening along the line. Here its always blazing fast with multiple php applications.
Maybe try xcache for awhile to see if its a php shared memory locking issue.
Maybe try xcache for awhile to see if its a php shared memory locking issue.