Hi, sorry for the delayed answer.
SAP GUI for Java in my experience is slightly, but not at all significantly slower than GUI for Windows. As I feel it is coming from the interpretation of the GUI commands. The execution of the commands seems to be the same.
The bigger pain is the missing functions, like graphical Print Preview, graphical Screen Painter.
I'm still on 7.30 rev 2, as I didn't want to install JDK...
Sz.