Do you happen to have Silverlight installed and have given paypal.com permission (or a blanket permission to every site) to access the plug-in? You can check in Safari preferences > security > Internet plug-ins. I recommend setting Silverlight access to ask or block.
I have. Just hangs there. Left it for an hour already and still just frozen on the page. Tried it on another MBP and loaded fine. Just unsure what I'm missing here.
Silverlight is broken in El Capitan beta 3 / public beta 1. You need to disable it. I don’t think that you need to use Silverlight for anything on Paypal.