Simple to do in MacOS (or any *NIX flavor), lots of different [free] options: pfctl / dnctl from terminal (optionally, use Throttle which just wraps some configuration and scripts around the aforementioned), you can snag the free additional XCode tools (not the full XCode), that include the Network Link Conditioner (if you want a UI solution).
All the above assumes a single machine (vs. doing it upstream at the router), and if it's just one machine, using a browser, as pointed out, Chrome dev tools makes it super easy as well.