Its important to look at packet loss as it is very often associated with slowdowns - in fact the tcp protocol is built on the assumption that if the network is to slow it should discard the packet as a hint to slow down transmission. The easiest tool to use is MTR, which combines traceroute and ping to show latencies and packet loss. There are a number of tools which can help with diagnosing the problems, but no single bullet. There are lots of possible causes of the problem - the most likely being bottlnecks between providers, or between points of presence in a providers network.
