Most of our public machines are Macs and we haven't seen that much of a delay with Popup. We have with Notify but were able to script the response with Notify so that if it didn't show up then the response to the Pharos server was a positive meaning that the student approved the print job and would send it to a release station. I would start by verifying that the OS X firewall is functioning correctly and test it off if you haven't already. You could then try to monitor the traffic with something like Wireshark to see if there are any glaring issues.
I have found that file size seems to play a role in the speed of the popup on both the windows and mac side of things. You may want to check the printer settings to see if the jobs are being processed on that local machine or if they are going to print for that process.