kubuntu V15.10 extremely sluggish to use, switching windows slooooow

[robert]

Hi there,

I did install kubuntu 15.10 on my ubuntu box.
Now working with it is very sluggish.
It sometimes takes 5-10 seconds when I click a window until it is 
activated. Or when I want to enter the subject of a mail it takes very 
long until the entered text is displayed.

But not always. Sometimes everything works nice and smoothly.

I need to say, that I installed over an existing ubuntu with unity, and 
I did keep the home.

thanks for your tips.

robert
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