[Zope] Site-Access-Performance-Problem ?

Hello, we have some strange problems with the "SiteAccess"-Product. We are building up three web-applications on one Zope-Server and three different domains. Everything works fine with the "Site-Access" but if you have more than 5 requests at one of the sites the speed gets very slow and you have to wait about 3-5 and more seconds to get back a simple dtml-document. We made some tests with the __no_before_traverse__ in the url and found that there isn't that speed-problem if we access the documents direct without use of the virtual-hosting. Has anybody the same problem or an idea how to solve this ? Martin