After upgrading my Debian unstable to Kde 4 a few months ago, the Flashplayer started to be very slow, especially in the full screen mode. Using Kde 3 I was able to watch full screen Youtube or IPlayer, but now the frame rate is somewhere around 10 fps. Is there a way to improve it?
There's a few things that could come into play here such as which browser you are using, which flash plugin it uses and whether you changed your video card drivers at the same time you upgraded to KDE 4.
I'd recommend switching browsers and seeing whether that helps (at least narrows down the problem). So if you use the built-in konqueror, then try firefox (or vice versa). My guess would be that firefox with the Adobe Flash Player would perform better then konqueror with the free flash player...
It is likely an issue with your drivers. I had a similar problem where KDE 3 would work fine with any Flash sites. Upgrading to KDE 4 had a similar problem to yours; it got very slow, especially the bigger the flash module.
In my case the computer has a UniChrome video card, and forcing it to use the drivers fixed the problem. I don't know why I didn't have to do the same in KDE 3, but it worked for 4.