standard products that probably will not help you. I'm not a speed guru, but load balancing with ZEO would probably be the easiest way of improving the performance in this case. That's my guess.
If he's got a dual cpu'd machine and is running two zeo instances, he's fine unless he's got a ram problem. To be running 2 zeo instances on the same machine, they have to be doing some type of load balancing or faking load balancing so that ea. zeo client can respond to requests. One thing that might help is CRANK up your on disk zeo cache as high as you can afford - it's just disk space. If this hasn't been done, the zeo clients might be spending a lot of time 'requesting' objects from the zeo storage server when you can be caching them on the zeo clients. Some substantial effort would likely be required to optimize the Plone installation. If you choose this route, look at the skins - there's lots, lots of layers, and lots of searches going on you could eliminate or rewrite to be more efficient. If you go down the Zope/MySQL path, this would require some substantial effort also. I would recommend, if you're going to be putting substantial effort into the application, look at a CMF/Five implementation, relying heavily on using the View paradigm Lennart references. Another option, not knowing your technical level, is to implement in Zope 3. I personally am liking Zope 3 alot. IIRC, there is a Zope 3 CMS project out there. Andrew Sawyers