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AFAIK that's because of the default theme being more complex. Change to a minimal theme without gradients, pixmaps and effects and it should be as fast as Qt 3.
A much better option is to use NX instead of plain old remote X or VNC though. It's so much faster it's not even in the same league.
Well, bringing up an image chip from a 4MP camera, selecting some lines and point measurements results in saturating a gigabit connection for .5s or longer bursts (~40mb/s). Doing the same thing in a fltk program I get acceptable performamce over a 1/4 T1 line internet connection (only image navigation is slow,measuring is faster). QT's problem: excessive chatting chatting between the server and client because of all the QGraphicsView stuff. Note this is a also problem with windows remote terminal.
Running vnc (or rdesktop) does help with this because all the composting is done on the server only and screen updates are transferred.
The easy answer has been to not use Qt but any number of non over engineered toolkits. Comments from customers is that the non qt stuff feels amazingly faster. And build management is much easier.