mousemove triggers more often with console open?

I'm experiencing something weird, but I'm not sure if it's intended or what's causing it.

I'm experimenting with some JavaScript that loops through a set of images when the user moves the mouse across the screen. I'm using jQuery mousemove for this. Here's a fiddle: https://jsfiddle.net/sy35dzeh/1/

The behaviour I'm experiencing is some kind of throttle with the mouse movement. I want every pixel moved to increment the pixelCount variable. But when moving the mouse in longer sweeps, it seems like the iteration reaches a limit. This causes the counter to increment faster when moving the mouse slowly as there are more "pixels" per movement added. I get that this might be how mousemove works, but on top of that it behaves differently when opening the developer tools.

When I open the developer tools in Chrome as try to move the mouse again, the iteration is a lot more rapid. This is the behaviour I want, when the iteration becomes slower on slower mouse movement and faster when moving the mouse faster.

Here's a video of the difference: https://streamable.com/okqql3

Any idea why it's different when I open the console and any idea how to make the mouse movement affect the iteration in the way I want it to behave (like when I have the developer tools open)?

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