If I were to get a portable charger (6000mah) and a 4 port USB hub, would the charging become slow? If I only plug in two devices, such as my iPhone 4s and my Tablet.
Nope, that's depend upon how many device you attached for charging, if you charging two devices the speed is accordingly that. but advising you to only connect maximum two devices not more than two coz its became really slow.