That's the nature of having vsync enabled. If your frame rate drops below 1/60th of a second, then it will have to wait for every other vertical refresh, which equals 30 fps. And if you drop below 1/30th, then it will make every 3rd refresh, which is 20 fps, and so on...
You can disable vsync with the viz.vsync() command. Your framerate will run as fast as possible, however you might notice tearing in the image.
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