Exact Duplicate: https://stackoverflow.com/questions/473465/firefox-plugin-to-simulate-slow-internet-connection
How do I simulate a slow internet connec
Apple now have an official developer tool called Network Link Conditioner which simulates different network conditions. It has some more powerful features that alternatives like SpeedLimit and Charlie — for example it can simulate packet loss and DNS delays as well as bandwidth and latency — and it is free.
Network Link Conditioner is provided with Xcode, with is a free download but is also a very large download!
Ref: https://apple.stackexchange.com/questions/24066/how-to-simulate-slow-internet-connections-on-the-mac