Thanks for the tipp Oshyan.
Amazingly, after I restarted the server today it has a strange speed-behavior!
Calculating GI Cache it's more then 2 times faster than my Core i7
Also rendering is faster now: 2/3 of my i7's speed...
But it gets really, really hot - especially the RAM...
Someone intersted in some sausages? -- there should be finished 2 pairs grilled on the server

Unfortunately I had to rearrange some details in the scene. After some changes in the terrain I forgot to check my populations (floating trees) and had to restart rendering... But now I'm happy with the meadow...
(at least I will not make any more changes)
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