We had the same issue, as we currently have 239 mods on the server, when I updated the server from 1.5.2 to 1.6.4 we had quite a bit of lag on NEI, so much I dreaded using it. In fixing another IC2 related issue I updated every single IC2 addon mod to their latest versions, and it completely went away. Not sure which mod was causing it as it was 5-6 mods updated at once, but it was definitely one of them.
My computer lags with just 19.
Might be since it's a rubbish old mac.