So I realize this is an old topic, but i ran into the same issue today and it took me forever to figure out why the Sphax texture pack was running so slow, when it had been working fine beforehand....
The long story short: I am using MCPatcher, and I uninstalled a Mod that wasn't working properly... and upon doing so I also (by accident) uninstalled the "Extended HD" mod that comes defualt with MCPatcher. So...the next time I log into minecraft after doing this the game is EXTREMELY laggy.
The FIX: After hours of tweaking and research, I simply re-patched the "Extended HD' mod that came with MCPatcher. And now the Sphax texture pack (128) works like a charm. Accidently un-patching that mod seems to have been my issue. Re-patching it fixed me.
So, I wanted to post my solution in case anyone else runs into the same thing... maybe google will lead them here and they can try this.