This isn't an issue with your PHP code. The webserver is responsible for accepting and threading off requests, so if it stalls on one script before loading a second, than the version of the webserver software is either not multithread capable or not configured to allow multiple requests.
If your referring to your local machine dev environment, than I wouldn't bother with configuring it.
If you are on a shared host, it should already be configured. If not, contact your host.
If you are on a VPS or dedicated, than you'll need to follow the documentation for your webserver software to set up the number of allowed requests.
header('HTTP/1.1 420 Enhance Your Calm');