Posting this in the hope it'll save someone else time/headaches.
TL;DR version: Use cached package installs if you want to reduce the amount of data transferred to your client computers. An immediate install verifies the package on the distribution point before downloading it, essentially doubling the amount of data being transferred. With cached package installs the verification occurs after the package is downloaded.
Observed with JSS 9.72, earlier versions not checked
Longer version...
I've got about 800 student MacBook Airs to upgrade to 10.10.4 in the next month. I originally set up a non-cached install policy to deploy the 10.10.4 combo update package thinking it would be the fastest way. That turns out to be wrong...
Since school is out for the summer, our wireless network isn't very busy, so I was surprised how long it took for my test machines to download and install the update. Firing up Activity Monitor to see what was happening on a student machine showed a network data received count much higher than I'd expected. Using Terminal on another machine to manually invoke 'sudo jamf policy' revealed that the package was being verified prior to being downloaded from my distribution point. When I changed to a policy using a cache package action the overhead of the verify before download was eliminated. Repeating the 'sudo jam policy' in to run the post-cache install cached package policy runs verified that the package verification still occurs before the package is installed (and it happens much faster verifying from the MBA's SSD rather than across the network).
