According to these blog posts from SUSE/Novell KDE developers that include bootcharts showing KDE startup, recent performance improvements in fontconfig and KDE/Qt have a noticeable effect on startup times. KDE can even match Xfce startup time (both 5 seconds on a 900MHz laptop) when some KDE features are turned off to match Xfce more closely in terms of functionality.
If you want to improve your KDE boot time, check out the blog post here. I find it takes more time to load the kernel than KDE, but that could be just me.