...and yet every other developer somehow manages to not use 100% CPU and still has responsive mouse input?

Christ, the game's 12 years old. You could render everything in software without touching the video driver and still not get to 100% on a modern processor core.

Use the command-line switches. Run it from the command-line or just change your shortcut to have
-graphics_fullscreen
at the end.

Failing that, you can also try
+graphics_screenheight 768 +graphics_screenwidth 1366

The focus-follows-mouse text boxes (rather than being able to click on them like every text-box ever) and the fact that you need to manually delete text like, "fill this in," are really stupid. The lack of copy-and-paste is a bit irritating, but that's made much worse by the fact that the ...

When running, this game take one of my CPU cores up to 100% and just sits there. There's no reason that a 12-year-old game justifiably needs that much compute time. By comparison, Serious Sam 3 takes about 40%.

I can only imagine that it's spamming the system clock, or something equally pointless.