srcds in single core, indeed, but only some components are multithreaded.
if you start a heavy populated single server on a multicore server, you will see load on more than one core, but not equally.
one core will be like 100% (main srcds) and the other one will be like 10-15% (particle effects and other things).
you will not see srcds running on 2 cores, 50% load on each core.
the problem is that the main srcds process won't run across multiple cores in multiple threads, that's why you will see only one core having load 100%, that's why you will face the same problem, that's why for best results you need a better
single thread performance processor