Data taking forever to get to printer - why?

When you say 'forever', that implies that the print job never completes. If the print job does complete, approximately how long does it take? 1 minute? 1 hour? Longer? Are you able to successfully print using a method other than AirPrint? Does the issue occur if you are right next to the printer when attempting to print using AirPrint?

What model Brother printer are you using? Have you confirmed that you have the latest Brother printer drivers installed for the version of macOS that you are running? This website provides the latest Brother printer drivers.

http://www.brother.com/apps/ps/en/

Steps you could also try include:

Deleting and then re-adding the printer from your Printers & Scanners System Preferences

Reset the NVRAM

Restart your Mac

I also suggest checking the Brother website to see if they have any firmware updates available for your printer. I have a Brother MFC laser printer and have not had any issues with AirPrint.

Hazel you might try resetting the printing system than re adding the printer If you can't print from your Mac or iOS device - Apple Support

Restarting your computer could result in faster printing, however have you tried reconnecting your printer or checking the connection between the printer and the computer?