Interesting, as I also run an xpSP3 dell lapper and have used OE, but mostly MS Outlook.
In any case, there was not an issue with CPU or memory use while using OE.

I’m not at my xp right now, so I can’t test the latest version of Avast (Premier) with OE.
(No issue with MS Outlook)
Will test later and see if I see unusual activity from Avast.