Photoshop :: CC Slow In Performance On Big Files?

I've been using PS CS4 since release and upgraded to CS6 Master Collection last year.Since my OS broke down some weeks ago (RAM broke), i gave Photoshop CC a try. At the same time I moved in new rooms and couldnt get my hands on the DVD of my CS6 resting somewhere at home...So I tried CC.
 
Right now im using it with some big files. Filesize is between 2GB and 7,5 GB max. (all PSB)Photoshop seem to run fast in the very beginning, but since a few days it's so unbelievable slow that I can't work properly. if it is caused by the growing files or some other issue with my machine.
 
The files contain a large amount of layers and Masks, nearly 280 layers in the biggest file. (mostly with masks)The images are 50 x 70 cm big  @ 300dpi.
 
When I try to make some brush-strokes on a layer-mask in the biggest file it takes 5-20 seconds for the brush to draw... I couldnt figure out why.And its not so much pretending on the brush-size as you may expect... even very small brushes (2-10 px) show this issue from time to time.
 
Also switching on and off masks (gradient maps, selective color or leves) takes ages to be displayed, sometimes more than 3 or 4 seconds.The same with panning around in the picture, zooming in and out or moving layers.It's nearly impossible to work on these files in time.never seen this on CS6.
 
Now I wonder if there's something wrong with PS or the OS. But: I've never been working with files this big before.In march I worked on some 5GB files with 150-200 layers in CS6, but it worked like a charm.
 
SystemSpecs:
 
I7 3930k (3,8 GHz)
Asus P9X79 Deluxe
64GB DDR3 1600Mhz Kingston HyperX
GTX 570
2x Corsair Force GT3 SSD
 
Wacom Intous 5 m Touch (I have some issues with the touch from time to time)
 
WIN 7 Ultimate 64
all systemupdates
newest drivers
PS CC
 
System and PS are running on the first SSD, scratch is on the second. Both are set to be used by PS.RAM is allocated by 79% to PS, cache is set to 5 or 6, protocol-objects are set to 70. I also tried different cache-sizes from 128k to 1024k, but it didn't work a lot. When I open the largest file, PS takes 20-23 GB of RAM.