Looking at the code of devicepx-jetpack.js it looks like devicepx-jetpack.js runs a function called 'zoomImages' every second. This function ensures that every image on your page looks great on retina displays.
The reason it runs every seconds is to make sure if new images are loaded to the browser, for example using the infinite scroll, will be displayed nicely on retina displays as well. The action is pretty fast and skips images that have already been resized as well as is disabled if you are not using a device with a high resolution retina display.
The graph that you are seeing I believe is not devicepx-jetpack.js being loaded every time but the inspector telling you that the browser is performing an action because of devicepx-jetpack.js, running the 'zoomImages' function.
I have created a github issue looking to look further at optimizing the devicepx-jetpack.js script. https://github.com/Automattic/jetpack/issues/820
Let me know if you have further questions or anything else you would like me to take a look.