Both Firefox[1] and Chrome[2] (and, presumably, Safari and Edge?) have a network throttle in their dev tools. Apart from that, you can use OS level throttling, e.g. Network Link Conditioner[3] for OS X. Couldn't see any incremental rendering on Fefes Blog though.

[1] https://blog.nightly.mozilla.org/2016/11/07/simulate-slow-co...

[2] https://developers.google.com/web/tools/chrome-devtools/netw...

[3] http://nshipster.com/network-link-conditioner/ (We use this to throttle Websocket transmissions, which, last I checked, the browser devmode throttles don't apply to.)