In slow internet situations you probably only want a couple of apps using your precious, precious bandwith. And disabling all background processes that use your bandwith is such a hassle (dropbox, backblaze, mac app store, you name it, all of them use your bandwith). Enter the excellent Tripmode app. Enable Tripmode and you select which apps are allowed to use your bandwith. Works like a charm.
Skyping? Use Tripmode to make sure you have the best audio quality.
Phone as hotspot/thethered? Use Tripmode to keep data usage low.
Horrible wifi? Use Tripmode to put all the bandwith to the thing you do want to get done.