via the Portal Speedrun Guide doc:

you can try: turning down antialiasing and filter; lowering your resolution; setting the game’s priority to High in Task Manager.

you can also try mat_dxlevel 80 in console even though texture quality barely affects frame rate according to the doc.

Also if you're using streamlabs, lowering the Output (Scaled) Resolution, changing downscale filter and using performance mode should help with CPU usage.