Graphics performance very bad with Radeon HD 5770 and Dual-Monitor setup

After a new installation on my old hardware, I noticed considerably worse performance, although the hardware was not altered. Overall operation seems fine. But if I start a youtube video and put it in full screen, I get horrible performance -- even several seconds of freeze. Also, running e.g. wesnoth is quite slow. I used to be able to run wesnoth on the big monitor while running a video on the second monitor simultaneously with no problem at all. My Graphics Card: $ sudo lspci|grep VGA 01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Juniper XT [Radeon HD 5770] My Screen Resolution(s): (output cropped a bit by some lower resolutions) $ xrandr Screen 0: minimum 320 x 200, current 3200 x 1080, maximum 8192 x 8192 DFP1 disconnected (normal left inverted right x axis y axis) DFP2 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 521mm x 293mm 1920x1080 60.0*+ 50.0 59.9 60.1 50.0 60.0 1600x1200 60.0 [...] DFP3 disconnected (normal left inverted right x axis y axis) DFP4 disconnected (normal left inverted right x axis y axis) CRT1 disconnected (normal left inverted right x axis y axis) CRT2 connected 1280x1024+1920+0 (normal left inverted right x axis y axis) 338mm x 270mm 1280x1024 60.0*+ 75.0 1280x960 60.0 [...] My Kernel and ATI drivers: sys-kernel/linux-sabayon-3.12.14 x11-drivers/ati-drivers-13.12#3.12.0-sabayon Direct rendering is enabled: $ glxinfo | grep rendering direct rendering: Yes When measuring performance with glxgears: $ glxgears 15868 frames in 5.0 seconds = 3173.469 FPS This is my xorg.conf, presumable generated by amdcccle (which I used for configuration) Section "ServerLayout" Identifier "Main Layout" Screen 0 "amdcccle-Screen-0" 0 0 EndSection Section "Module" SubSection "extmod" Option "omit xfree86-dga" EndSubSection Load "i2c" Load "ddc" Load "vbe" Load "dri" Load "glx" Load "synaptics" EndSection Section "ServerFlags" Option "AllowMouseOpenFail" "true" EndSection Section "Monitor" Identifier "Generic Monitor" HorizSync 28.0 - 80.0 VertRefresh 43.0 - 60.0 EndSection Section "Monitor" Identifier "0-DFP2" Option "VendorName" "ATI Proprietary Driver" Option "ModelName" "Generic Autodetecting Monitor" Option "DPMS" "true" Option "PreferredMode" "1920x1080" Option "TargetRefresh" "60" Option "Position" "0 0" Option "Rotate" "normal" Option "Disable" "false" EndSection Section "Monitor" Identifier "0-CRT2" Option "VendorName" "ATI Proprietary Driver" Option "ModelName" "Generic Autodetecting Monitor" Option "DPMS" "true" Option "PreferredMode" "1280x1024" Option "TargetRefresh" "60" Option "Position" "1920 0" Option "Rotate" "normal" Option "Disable" "false" EndSection Section "Device" #BusID "PCI:01:00" #Option "RenderAccel" "on" #Option "XAANoOffscreenPixmaps" #Option "BusType" "PCI" #Option "ColorTiling" "on" #Option "EnablePageFlip" "on" # UseEvents is causing segmentation faults with # NVIDIA 6xxx, 7xxx and >=275.xx.xx drivers #Option "UseEvents" "True" Identifier "SabayonVga0" Driver "fglrx" Option "LogoPath" "/usr/share/backgrounds/sabayonlinux-nvidia.png" EndSection Section "Device" Identifier "amdcccle-Device-0" Driver "fglrx" Option "Monitor-DFP2" "0-DFP2" Option "Monitor-CRT2" "0-CRT2" BusID "PCI:1:0:0" EndSection Section "Screen" #Option "AddARGBGLXVisuals" "true" #Option "RegistryDwords" "EnableBrightnessControl=1" Identifier "Screen 0" Device "SabayonVga0" Monitor "Generic Monitor" DefaultDepth 24 SubSection "Display" #Modes "1024x768" "800x600" "640x480" Viewport 0 0 Depth 8 EndSubSection SubSection "Display" #Modes "1024x768" "800x600" "640x480" Viewport 0 0 Depth 16 EndSubSection SubSection "Display" #Modes "1024x768" "800x600" "640x480" Viewport 0 0 Depth 24 EndSubSection EndSection Section "Screen" Identifier "amdcccle-Screen-0" Device "amdcccle-Device-0" DefaultDepth 24 SubSection "Display" Viewport 0 0 Depth 24 EndSubSection EndSection Section "DRI" Mode 0666 EndSection