Some time back, Google Chrome introduced network link conditioning, or what we know mostly as network throttling in the browser. This is a feature that we've come to know and use in Webpagetest, but until recently wasn't available in the devtools.
We know that replicating the various conditions that web apps have to suffer through is a top priority, and a reflection of the urgency to develop for mobile performance. Imagine spending your days wearing comfy and flexible sweatpants (no throttling), and one day being fitted into some constrictive denim, or even skinny jeans (throttling). You suddenly see the (page) weight you've put on. lol.
After some discussion, network throttling is now available in Firefox Nightly, and in Nightly only.
This feature appears whilst in responsive design mode (for just reasons), and has a number of classic settings to help developers replicate slow networks and determine how files are loading, as many of the comments have indicated.