ZopeMultiThreadedness

cut here

Using ZServer, Zope will always run multi-threaded. In the olden days, before ZServer, Zope ran single threaded through PCGI or a simple HTTP server that is no longer bundled.

You could still use Zope this way, that is, use it without ZServer, and have PCGI go directly to Zope. This would indeed be single-threaded. But why would you want to do that? taken from MP's response on mailing list.

cut here

ZOPE has no threading problems. Its fully threaded but its networking services in ZServer do multiplexing over select() using asyncore. You can run multiple ZEO instances on a machine if you want to bind specific ZOPE instances to a processor. (requires FreeBSD?)