Fast in theory is not always fast in practice. I am on Bigpond Cable which is uncapped in both directions allowing a theoretical 8000kbs either way. It's not always that fast though. Bigpond is a subsidiary of Telstra, Australia's largest (by far) phone/Net company and as such has HUGE capacity and speed with many 'dark' (not used) lines so there is also heaps in reserve. Internet traffic is subject to other things however, not just what the ISP will let you do and what speed they let you do it at, but also the amount of customers they have, what those customers are collectively doing, whether the ISP is a wholesaler (sells bandwidth to other smaller ISPs) as well as a retailer and what else the network is used for (in my case Pay TV as well as the Net).
Customers on your new ISP may be using more bandwidth which is always going to slow things down, even a little. Activity does not have to be illegal to consume bandwidth but quite often the bandwidth hoggers are doing exactly that - mass emailing people and filesharing copyright material are two of many big consumers of bandwidth.
One other thing is that ISPs don't benchmark their networks while they are under big loads. To do so is not good business sense because under load there is always a lesser result. This is why they all say "Our network can go this fast" in big lettering and then on the back page in 1pt lettering the disclaimer says something like "Theoretical network performance".
Caveat Emptor.
