Interestingly enough, VM actually seem to want to know about slow browsing speeds - that was the opinion of the level 2 tech who gave me a ringback once when I had poor speeds. If it's a consistently low speed which would be indicative of a network misconfiguration then obviously that's the likely cause, but if it's unpredictable speeds (the kind which result in 'spiky' graphs on bandwidth meter programs) then that's the kind of thing they're interested in, because it's obviously an indicator that something's a little screwy upstream.
It's also the kind of problems I've been having, and I'd put money on it being overcontention - but if they don't give me the speeds I'm paying for then I'm just gonna scream blue murder until they sort things out
because there's no excuse for not delivering 20Mb over a cable connection, line distance just doesn't factor. Sure physical cable faults can come into it but again, that's their problem not mine.
I think if you have a problem and it's acknowledged by VM, you just keep plugging away and eventually you might see some success. It's what I'm going to do if problems persist.