[Users] xlog performance

I recently did some performance testing with openser 1.1 and stumbled over a rather strange thing. The xlog command from the xlog module seems to cause a huge performance penalty. I have a rather complex config with many routing blocks and mysql queries with which I can achieve up to 100 calls per second (authenticated INVITEs) per openser instance. As soon as I add *one* xlog statement to the config the cps rate goes down to 25, and the average SIP message response time goes up from <10ms to >200ms. Has anyone else observed this behavior or could this issue be caused by a misconfiguration? thanks, Christian