I've run the ICSI netalyzr (see http://www.icsi.berkeley.edu/ ) which is available at http://www.bit.ly/sn345 .

It was mentioned by Steve Gibson in Security Now episode 345 ( http://www.grc.com/sn/sn-345.htm ).

It uses Java so you need Java to be installed.

Netalyzr finds a rather large uplink buffer size (from my pc to the internet):

Network buffer measurements (?): Uplink 4800 ms, Downlink is good –
We estimate your uplink as having 4800 msec of buffering. This is quite high, and you may experience substantial disruption to your network performance when performing interactive tasks such as web-surfing while simultaneously conducting large uploads. With such a buffer, real-time applications such as games or audio chat can work quite poorly when conducting large uploads at the same time.

I'm wondering if this is due to my modem (vigor 2820n).

2850n here and I get the same results from that web site. They infer that a large buffer will slow your connection down, as does Steve Gibson in his podcast, and I can find no fault in his explanation of the effect. The previous router, a link sys WAG54GS (ver1.1), reports a slower line speed but appears faster in loading web pages. The Berkley University web site reports less than 300ms buffering for it.
Has anybody, any idea how to reduce the buffer capacity, I can find no mention in the instructions, or can this only be done with a software upgrade?