Thank you very much for your answer.
I know this article very well – its bookmarked and part of the process of every website I produce with Theme X.
I already installed W3 TOTAL CACHE, made GZip optimiziations and compressed the images.
Now I installed two different caching plugins but there is a performance issue without this optimizations.
I already used pingdom, gtmetrics, pagespeed to mesure this issue.
I use the plugin Query Monitor and it is showing often slow querys.
I dont have Problems like that with my other Theme X websites at the same hosting provider.
It would be great if you could have a closer look in the admin area.
Hope you could help.