Overhead of the operating system and more likely a massive amount of packets per second will easily peg a single core.. I did some tests with nginx (comparable to node.js) and it easily pegged a xenon 2 cpu quad core 8GB ram (all 8 cpu's were 90+%) with a paltry 8055.77 rps over 2 x 10gbit ethernet but then this is more likely an OS / fine tuning limitation