Hi,
I use "Enable per-user bandwidth restriction" (checked) - using these settings: 2500 (Kbits upload) and 700 (Kbits download).
It works pretty well for me, when I test speed limits with my iDevice.
Slow portal behavior seems more a DNS issue to me.
I remember seeing that when add MAC on the "pass-through-MAC" page, the speed settings over there aren't taken in account (bug ?!).