Php only serving one page at a time [fix!]
After years of confusion I finally found the solution to why php was only serving one page at a time.
When you execute session_start() the session array for that user gets locked to prevent concurrent write errors. Until the script finishes or you run session_write_close() any other script that tries to access that session will stall.
So, the fix is to run session_write_close() after you are done writing session data but before anything time consuming (large shell_exec()s, sql querys, downloads).