The video shows it's clearly a rendering issue.

To be a router/ISP issue, the browser would have to be fetching images over the network every time you scroll to that part of the page (unlikely).

Or something perverse like the router is flooding the N900 with pings to slow it down

Meaning?

It could be, and I'd be very interested to see someone else's N900 visiting the same sites.

It could also be the RAM is too full and it's swapping. We haven't been told how many other apps it's running, or what their memory usage is.