That's an annoying problem, it only occurs in KDE Plasma, with fglrx drivers, also changing the desktop compositor from kWIn to openbox. I'm not having this problem with the open source drivers.
Basically, opening a game (Team fortress 2 in this case) and then alt+tab and then clicking the taskbar icon to return to it, causes a huge performance drop and input lag in the game. About from 300~fps to 150~
With just using alt+tab when returning to it the problem does not occur.
If I click the game from the taskbar once, alt+tab again to exit from it, and return to it also with alt+tab still the same problem. (That means that the problem occurs after i pressed once the icon in the taskbar to return to it, also if I alt+tab again still the same problem, unless I kill the "plasmashell" process. Here we return back to the beginning.)
That problem does not seem to be kwin/desktop compositor releated.

Reproducible: Always

Steps to Reproduce:
1.Install fglrx drivers
2.Run an opengl game (I was using Team Fortress 2 through Steam)
3.Alt+Tab
4.click on the game icon in the taskbar to return to it


Actual Results:  
The frame-per-second of the game dropped from 300~fps to 150~ 

Expected Results:  
The performance should be the same.

Arch Linux, latest version
Latest catalyst-test package from the AUR (also tried with other catalysts)