Are you using the ray-traced renderer or the standard renderer? (The ray-traced renderer an obsolete way of making 3d items and not for normal AE usage, so if you're using Element or Cineware to do your 3d stuff, use the standard renderer).
CC 2015 doesn't have multiprocessing, so, if you're using the standard renderer try opening your project file in CC 2014 and render using multiprocessing. (Assuming you have enough RAM.)
Speaking of having enough RAM, what are your system specs?
What is in your scene?
Hej Szalam, I'm working on a pretty new:
GRAPHIC NVIDIA GeForceGT750M 1GB GDDR5 PROCESSOR 3.1GHz Quad-core Intel Core i7 MEMORY 16GB 1600MHz DDR3 SDRAM-2X8GB HARD DRIVE 1TB Fusion Drive
So I can't see that there is a ram issue. But I'm working with big 4k files...well
hmm, I still don't get it that AE doesn't find the right GPU...
Do you have any ideas how to fix that?
You didn't answer my question about whether or not you're using the obsolete ray-traced renderer and that's a pretty important question.
AE uses the GPU for very little, so, unless you're making 3d things with the ray-traced renderer, the GPU support doesn't much matter. More info here: GPU (CUDA, OpenGL) features in After Effects CS6 and After Effects CC
If you ARE using the ray-traced renderer, it will be slow if AE doesn't recognize the GPU. That GPU isn't on the list of "approved" cards (System requirements | After Effects), but you should have an option in the preferences to use "unsupported" cards. Again, this doesn't matter if you're not using the ray-traced renderer to make 3d things.
You also didn't answer my question about what sort of things are in your composition. Some things just take a while to render!