Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
KDE 4.9 will be released in August and overall it feels more responsive and faster during desktop use than its predecessors.
Aside from Qt Quick support in KDE Plasma Workspaces, improvements to the Dolphin file manager, and other changes, performance was also a focus for this six-month update to the KDE desktop. There's many performance improvements and bug-fixes to improve the user experience.
I've heard good comments about speed improvements of the Plasma desktop within KDE SC 4.9, so I tried it out for the past few days -- comparing Ubuntu 12.04 with KDE 4.8 to the latest KDE 4.8.90 packages. Yes, indeed, KDE 4.9 feels much nicer!