There really is very little excuse for it, throttling is built right into the dev tools of both Chromium and Firefox, it's a couple of clicks away. It's as important as cross browser testing and easier to do.
Another thing I've noticed a lot of sites are very poorly optimised for is latency. HTTP 1.1 is already poor in this regard due to the number of round trips required for setup, and when you start chaining these together latency becomes highly magnified, especially if they aren't on the same domain and you need a whole new HTTP setup for each step in the chain. Even with HTTP 1.1 this can be minimised by looking at the chain of requests and reorganising them to be less dependent on each other.
When your latency is in the single digits and it get's multiplied by 10, that's at most 100ms, now go to mobile and it's 400ms, now in the worst case a low bandwidth connection like ADSL (which will affect latency when the weight of each request is high) or a distant connection on a different continent and you can easily get into 10s of seconds.
I find this one the most annoying because the fundamental problem is poor utilisation of the connection, they don't even need to change the size of the payload, there are simply long periods when the browser is waiting to find out what to do next and the connection is mostly idle.
Another thing I've noticed a lot of sites are very poorly optimised for is latency. HTTP 1.1 is already poor in this regard due to the number of round trips required for setup, and when you start chaining these together latency becomes highly magnified, especially if they aren't on the same domain and you need a whole new HTTP setup for each step in the chain. Even with HTTP 1.1 this can be minimised by looking at the chain of requests and reorganising them to be less dependent on each other.
When your latency is in the single digits and it get's multiplied by 10, that's at most 100ms, now go to mobile and it's 400ms, now in the worst case a low bandwidth connection like ADSL (which will affect latency when the weight of each request is high) or a distant connection on a different continent and you can easily get into 10s of seconds.
I find this one the most annoying because the fundamental problem is poor utilisation of the connection, they don't even need to change the size of the payload, there are simply long periods when the browser is waiting to find out what to do next and the connection is mostly idle.