Some way to guess the speed of client connection

This is a great question. I haven't run across any techniques for estimating client speed from a browser before. I do have an idea, though; I haven't put more than a couple minutes of thought into this, but hopefully it'll give you some ideas. Also, please forgive my verbosity:

First, there are two things to consider when dealing with client-server performance: throughput and latency. Generally, a mobile client is going to have low bandwidth (and therefore low throughput) compared to a desktop client. Additionally, the mobile client's connection may be more error prone and therefore have higher latency. However, in my limited experience, high latency does not mean low throughput. Conversely, low latency does not mean high throughput.

Thus, you may need to distinguish between latency and throughput. Suppose the client sends a timestamp (let's call it "A") with each HTTP request and the server simply echos it back. The client can then subtract this returned timestamp with its current time to estimate how long it took the request to make the round trip. This time includes almost everything, including network latency, and the time it took the server to fully receive your request.

Now, suppose the server sends back the timestamp "A" first in the response headers before sending the entire response body. Also assume you can incrementally read the server's response (e.g. nonblocking IO. There are a variety of ways to do this.) This means you can get your echoed timestamp before reading the server response. At this point, the client time "B" minus the request timestamp "A" is an approximation of your latency. Save this, along with the client time "B".

Once you've finished reading the response, the amount of data in the response body divided by the new client time "C" minus the previous client time "B" is an approximation of your throughput. For example, suppose C - B = 100ms, and you've read 100kb of data, then your throughput is 1

Is this something you can discover on the client side? If you need to bypass proxies, then you can always discover the connection type on the client side and send that back. Another method would be to download a file on the client side via some scripted mechanism, record the bytes per second, and return that information to the server side.