...What graphics drivers are you using?
Are you running the distros in a virtual machine, from a LiveCD or straight off of your hard drive? The former two will always be slower. In that case I'd suggest running something like openbox which is light weight and eat less resources than full DE's. If it helps, I'm running ArchLinux with WMII, but even under KDE 4.4 it didn't take "a few seconds" for menu items to load. [edit] I don't think it has anything to do with the graphics drivers. I just downgraded to vesa drivers and Qt was still as responsive as ever.