I gave up on Jamf Admin a few years ago because it seemed to be less and less reliable uploading files to JCDS as compared to using the JSS Console Packages page for uploads. I don't have any speed comparisons between the two, but I don't have any complaints on the web interface performance.

This is why I began using the Jamf Admin app in the first place, it was painfully slow.

But back to the web interface I go, I guess...

As a norm I tend to use Google Chrome, it can be slow to upload. Swapping to Safari or Firefox can be a lot faster, it also fixes those long Availability Pending waits.
Only thing I use Admin for now is Printers and keeping our in house distribution server up to date.