I have said a few words about our apache performance before. You could see the result, that it may have taken a while until the http request was served if it was very crowded. In the end, that also did not help anymore and the performance was very poor. Actually, I have been spammed with mails from our monitoring system telling that the server load is very high.
Finally, we have moved to our new server last night. It smoothly serves the requests now and still leaves room for more requests.