A company I used to work for had a slow DSL line, with a ~7 year old pc, that they'd use to test their web-apps. That way, if it was responsive on that, then it'd be responsive on the customer's pc. I wish more companies tried this, instead of pushing the limit of what their expensive work-provided desktops could do.
Facebook has been experimenting with "2G Tuesdays", providing an option for developers to test using the site with slow / laggy internet connections:
http://www.theverge.com/2015/10/28/9625062/facebook-2g-tuesd...
This guy has a good write up on using netem to impair a connection to simulate a wan connection.
Pretty handy for testing. I've used it to simulate a bad uplink in between a dslam and bras.
https://www.excentis.com/blog/use-linux-traffic-control-impa...
I was going to recommend traffic manipulation and traffic shaping with netem and tc. I set this up at my last employer in the lab for use with our networked IP radios and such. Stimulating poor network conditions has been a mainstay in my testing ever since, even now that I work on web apps and distributed services.
And I tell them, "Exactly. If I make my app run well on this laptop, I know for sure it will run acceptably on every machine here."