From the little I know, Qt4 would actually be faster if the apps were not more heavyweight themselves. Or at least that's what I've heard around the time it was introduced. There are also other advantages like better rendering when resizing windows and multithreaded performance.
I guess the only way to know for sure if memory usage is lower, and performance is better, is to port parts of the code and test. I can't seem to find benchmarks but KDE 4 now is quite slimmer than what it used to be, even though it still uses more memory than KDE 3/Trinity.

Best regards,
Tiago

On Wed, Dec 1, 2010 at 9:01 AM, Mikle Krutov <nekoexmachina@gmail.com> wrote:
Hello, list!
Question is in subj of the message.
Because as for me all (i mean, all, not only kde4)  qt4 apps are much
slower then they were on qt3. So, what are advantages of migration?


Old mercenaries never die. They go to hell and regroup.

With best regards, Mikle Krutov, Bercut ltd. Technical Support department