internet slow, but tjener not busy

[Nigel Barker <tech@hiroshima-is.ac.jp>]

Hi All,

I've been noticing that when the lab is just moderately busy, say when
the Maths teacher brings in a small class to do Khan Academy, my
iceweasel is very slow. Even hovering on a link takes several seconds
before the pointer turns to a hand, and a click takes equally as long
before the wait cursor appears.Then pages load slowly.

Non-internet use is not affected - I can work as normal and my
computer is responsive, files open and save quickly.

When I look at the load on tjener, it is less than 2 (with 4 cores).

The firewall (Untangle) is telling me its cpu load is small, and it
has more than half a gig of memory free.

Where should I be looking to locate the problem?

Thanks

Nigel