Could your try to reduce the resolution once and tell us what happens?
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Hi thank you for your tips! Logging off from iCloud totally and quitting Photos and "Slimbatterymonitor" (add-on to show battery) the fonts were not blurry anymore!!
While testing things out I found out the following things:
1. Photos and "Slimbatterymonitor" are showed as "consumes high GPU" in the Activity processor. So this explains somehow that they caused the fonts to be blurry in a lower resolution.
2. When I only run Photos in the lower resolution, plugging off the magsafe resulted in the fonts not being blurry again. Plugging the magsafe in again and Photos still running resulted in fonts being blurry.
I don't use Photos app anyway, so finally I can use my MBP in a lower resolution again.
But now my real problem comes!
3. My problem is: When I connect an external monitor through the mini display port to my mac which is on the lower resolution, while my fonts are ok (Photos.app and slimbatterymonitor is closed), the fonts start to be blurry again after connection it to the external monitor. But the Fonts are only blurry on the MacBook pro window not on the external monitor. As soon as I switch to standard resolution the fonts are okay on my MBP while being connected to the external monitor. If I plug off the mini DVI, the fonts on my MBP are not blurry anymore, while being in a lower resolution. Do you have an other Tip for blurry fonts on my MPB while being connected to an external monitor? This would help me and my setup a lot. In El Capitan and before I could use my MPB in a lower resolution while being connected to the same External Monitor (same cables etc.) without any problems regarding the fonts.