Everyone has fast dev machines.
We access the site through a local domain name like orbs-sunny.com referring to my personal computer; however, this is routed through Nginx with throttling on to simulate a slower external connection.
If we need a faster connection for certain types of testing like uploading 100MB files, we use orbs-sunny-fast.com. This goes directly to our machine and skips nginx.
It is not perfect in that it doesn't simulate a slow computer; however, it's good enough to make everybody pay attention to speed.
We did this because speed related issues were often ignored at the time of original development. We found that the slowdown on execution hasn't slowed development since our app is supposed to be responsive anyway. If it's not responsive, it's much better to fix it while your head is in that space. This results in faster overall development.
Sunny