I've setup my RPi Zero W as a Wireless Print Server (with cups, samba packages) for an old HP LaserJet and it works great. Prints from my Windows laptop are nearly instantaneous.
I then installed avahi-discover package to enabled AirPrint and that works as well. I can print from my iPhone. The phone automatically discovered it.
The problem is that an identical print from an iPhone takes around 5-6 minutes before it's even sent to the printer. The reason is that with Windows all the heavy lifting of processing the print is done on the computer with the installed printer driver. With AirPrint, it seems that it offloads that to the RPi.
Is there anything I can do to speed up the prints from my iPhone?