First, go to the Nvidia Control Panel and try to disable "Threading Optimization" (might be spelled slightly different; I'm using localized drivers) in the 3D settings. On my GTX 680 this option causes even empty/simple drawings (at least in OpenGL) to sometimes utilize 100% CPU load (at least for one core).
Once this is done (and there aren't any noticeable improvements), try to reduce things your CPU has to handle. Especially for onboard sound cards too high quality sound settings can have a negative impact on performance.
It's been a while since I've tried HoN, so bear with me if the following options aren't there:
- Try to reduce your vision range, even if it's just by zooming in a bit.
- Disable UI elements you don't need.
- Enable level of detail or lower the model (or environment) detail in general. This is drawn by the graphics card, but the CPU still has to upload/modify it from time to time.
- If you're running on an onboard sound card from Realtek, try to lower the maximum number of sound channels to 16 or 32 and disable hardware acceleration (even if available; the drivers lie about some settings in some cases and emulate missing things in software on the CPU).
More generic suggestions:
- Try playing without sound.
- If you're running any programs that provide game overlays or similar tools (e.g. Messengers such as ICQ, Trillian, etc.), disable/close them before launching the game.
- If possible, use a standalone sound card (PCI or PCI-E) with proper hardware acceleration.