Actually it does make perfect sense now. For some reason, people who connect directly to a cable service so that the external interface of the core is a public address and exposed to their ISP, are (sometimes?) getting their external interface set to an MTU of 576 instead of a more reasonable number like 1500. This dramatically impacts performance.
It isn't clear to me whether this is because the ISP is sending this (legacy) value to Kubuntu and Kubuntu is simply complying with it, or whether Kubuntu is in some way deriving this information, but it does seem to be new behaviour (post 0710 at least). Manually correcting it corrects the speed issue. I have asked a forum poster experiencing this to research it a little more to see if we can determine what causes it and if there is a deterministic way of correcting it during install...
