Wow, quite a response from a simple observation, thanks for the feedback.
So, for what it's worth, I'm running XP sp3, Comodo firewall (free) and Firefox (latest releases of all); I regularly run MalwareBytes and CCleaner to hopefully optimise things.
As stated, over the course of several days I tried various Avast settings including Vladimyr's suggestion of "intelligent stream scanning" both on and off.
But DavidR's comment confuses me ..... "It is hardly surprising as if you have the free avira ..... you have no web protection" ..... that's not the impression I got from the interface; if there's no "web protection", why's there a tick box for WebShield and why'd it cause such a slowdown?
I'm not trying to be funny, but as a non-expert on PCs/networks, Schmidthouse's comment on Firewall, Malware, OS config & Processing speed affecting (download?) speed is logical, but surely their effect should be somewhat similar between packages and I'd have thought the fewer system resources required implies a more efficient package.
As I said, it's a shame 'cos I liked the look/feel/tweakability of Avast.
Sadly, unless there's an EASY resolution to this issue I'll have to stick with Avira (with its flaky update procedure and annoying pop-ups).