For your need to simulate a 256kbps connection, two things have worked for me. Both require a good few mbps connection to begin with though:

1. Use Chrome's device emulator mode. You can ignore all other settings like screen size, user agent, etc. and just use the speed restriction. 2. Use a specialized WiFi router. I use the ones from TP-Link in which I can restrict speed for a particular device IP. DD WRT based routers can also help here.