Unity on 14.04 very sluggish on high CPU loads

[mod16]

First of all: I know this is no Handbrake issue, but I'm still asking here, because I hope to find more people having the same problem with Ubuntu 14.04 (Trusty).

Using Ubuntu 13.10 (Saucy) even while a Handbrake job was running - using all CPU cores - I had a very fluid GUI, now slow-downs or anything like that. So using the system i.e. for Browsing or stuff like that while Handbrake was encoding wasn't a problem at all.

Now with Ubuntu 14.04 this fundamentally changed. As soon as the CPU is under high load the GUI gets very sluggish and unresponsive. Even writing a simple text is delayed. Switching workspaces is working in slow motion only etc... As soon as the CPU load drops, everything is back to normal again (smooth and fluid).

I did a clean install of 14.04 - only kept my /home Partiton.

System is a AMD FX 8350 @ 4 GHz, Graphics are Radeon 5850 (using the default radeon open-source driver).

I already tried kernel 3.14 instead of the default 3.13 - but that made it even worse.
I also wanted to try the fglrx binary driver from AMD - but unfortunately this doesn't work at all - because the install always fails (missing ati/amdconfig symlinks and it complains card is not support (although it is) and stuff like that - tried the Ubuntu fglrx and the Catalyst 14.04 from the AMD website - always the same problem). But I think it's more of a Unity or Compiz issue, not a driver issue.

EDIT: typo

[mduell]

Existing thread you somehow missed when you searched before posting.

[mod16]

Oh, sorry!

In this case, I will of course use the already existing thread.

Thank you!

(Normally I really do search before posting )