What does the "Latency" field mean?
This number gives the time that it took to fetch a very small, known-to-be-good html document through the particular proxy the last time it was checked. This value is mesured in milliseconds.
The test document is less than 1KB in size, and the elapsed time is measured from the moment the connection is made until the moment the document has finished downloading.
The latency data is extremely subjective, since it's a reflection of our connectivity to a proxy, not yours.
Generally speaking, though, proxies with lower latency should be more responsive.