Right now server resource packs are almost useless.
Many players prefer to not use them because they "freeze". It's a few seconds on high end PCs (where it should take no time at all), and it can be almost half a minute on lower end PCs, which the majority of players have.
I really hope the issue gets reopened and the loading improved. Reloading only the assets changed would be a way to do that, and maybe they could try to prepare them in another thread (checking what textures must be kept, packing them in a single image, etc...) and only after that apply them using the main thread.
https://www.reddit.com/r/minecraftsuggestions/comments/6wc724/server_resource_packs_shouldnt_freeze_the_game/