It varies, these proxies come from all over. Some are faster, some are slower. They won't last more than a few hours, but it really shouldn't matter, as you get a new list of fresh proxies every hour from my service.
I can say that we test them pretty aggressively- if a proxy doesn't respond within a few seconds (timeout is 5 right now), it's marked as dead and not added to the list. So, the slowest proxies are eliminated.
If you PM me I can give you a sample to try out.