Not necessarily your ISP's fault. Try doing the test to a few more distant servers located in say Seattle and California. You might find that it is just the pipe between your ISP and the server you're testing against that is the problem. Not saying that DSL can't be oversubscribed as well - if an ISP has less bandwidth to the outside world than there is demand for that bandwidth, everyone will slow down even if they aren't shared on a neighborhood basis like cable is. That is much easier to fix that an oversubscribed cable node, but it still costs real money to do and a provider trying to cut costs may not want to make the investment necessary if the beancounters don't think they'll get a return on it in the form of greater customer retention (i.e., if your local cable company sucks, your local telco has less incentive to not suck)
You make some good points, but I tend to put it on the ISP more. Sure when an ATM goes down it isn't the ISP's fault and everything slows down. For an ISP to have bandwidth problems "outside" seems rare, as the bandwidth is so large that it would take so many users to all be trying to max their usages.
It does come down to how well the ISP manages their network.
Cable seems to be more prone to overloading a node, as this is the least bandwidth making it easier.
I saw this yesterday with Compcrap, where one min it tested 0.8 Mb/s, and then 5.2 Mb/s, and then 3.2 Mb/s, and finally 2.4 Mb/s.
This node is way overloaded.
One way to confirm the performance of your internet connection is to go to a site like http://speedtest.net/
The site tests your ping times, upload and download speeds and helps you cut through the "turbo," "ultra", etc marketing hype of your ISP to find out what you're really getting.
I wouldn't be so quick to say "confirm" the performance. ISPs can do some underhanded things.
I had speed problems with an old cable ISP.
The node was so over subscribed, that I could see when my neighbors went to church and came home. [no joke]
I worked with them for a long time trying to sort this out.
The first thing they did after showing the tech the speedtest results was to configure their end so speedtest ALWAYS reported full speed, yet the only time I had it was at 2 AM and 2PM could be hard just to get email.
I'm not says all ISPs are this underhanded, but that speedtest doesn't always give you the "whole story". Timing a download and comparing the size of large files will be a better "test".
The last weekend I was on this ISP, it was taking 18+ hours for SD On Demand, and DSL had just come into the area so I had it too and the same On Demand was less than an hour.
Several years later, the cable ISP finally ran another fiber to split the node.