I had a lot of problems getting a redirect to work, after which my script was intended to keep working in the background. The redirect to another page of my site simply would only work once the original page had finished processing.
I finally found out what was wrong:
The session only gets closed by PHP at the very end of the script, and since access to the session data is locked to prevent more than one page writing to it simultaneously, the new page cannot load until the original processing has finished.
Close the session manually when redirecting using session_write_close():
$strURL = "PUT YOUR REDIRCT HERE";
header("Location: $strURL", true);
header("Connection: close", true);
header("Content-Length: 0", true);
Make sure that your script doesn't write to the session after session_write_close(), i.e. in your background processing code. That won't work. Also avoid reading, remember, the next script may already have modified the data.
So try to read out the data you need prior to redirecting.