How do I create artificial lagging for testing?

My game react weirdly when the framerate drops. I'm working on some fixes to avoid this. The problem is: it's difficult for me to test how the game react when the framerate drops because the game always runs on 60fps on my machine.

Would any of you know of a way to artificially crate lags for testing purpose? Sometime more efficient than "running other games in the background" :P

If there's a way in construct itself for testing that, that would be awesome. Otherwise, I'm open to more "creative" solutions like spawning 1 bazillions sprites in the background or stuff like that. Anything that would be fairly consistent.

HTML5 uses V-sync as default, sometimes it would be choppy performance.

There no way to create artifical lagging until you use old computer with fullscreen doesn't make fixed resolution.

Stick in a particles object going full blast.

Stick in a particles object going full blast.

^^^This.