While it doesn't support multi-thread. You can assign a specific core to the HLDS process. Read up on CPU affinity and as it pertains to your particular build of Linux. HLDS defaults on core 0.
With no multi thread support it wont offload to the others like your OS does. So what you can do at the very least, is to keep HLDS from being shared with the normal OS operations by setting it to an affinity other than 0.
This will free up some processing power for you HLDS. ( Best we can do here )
Once done make sure to renice the HLDS process so it takes processing priority over the affinity you set it to.
Hope you find this helpful.
Processor affinity, or CPU pinning or "cache affinity", enables the binding and unbinding of a process or a thread to a central processing unit or a range of CPUs, so that the process or thread will execute only on the designated CPU or CPUs rather than any CPU.