@MigNov: You have noticed that the last post of this thread is over 2 years old ? The php-virt stuff looks cool anyway....
You should verify what program has the maximum CPU load under top. If Xorg shows an increase when you scale up videos this pretty much sounds to me as hardware acceleration for X is not playing nicely. I just set up an Manjaro based system with a X300 some days ago and didn't see any issue. I have switched to a GF 8600GT and Trisquel.