Magic trick for Kubuntu users:
1. Make an empty new directory ~/.compose-cache
2. Start a KDE application which has a text input widget (anything with a line edit or editable combo box will do)
3. Check ~/.compose-cache, it should now have a file in it whoose name is a long string of numbers
All being well, your Qt/KDE/Gtk applications should now start up 50-150ms faster.
Users of other distros are welcome to give it a try, I have only been able to test directly on Kubuntu. If you have a non-empty /var/cache/libx11/compose folder (eg. SuSE users) then this optimization is already enabled so you don't need to do anything.
For those curious about what is going on here, this enables an optimization which Lubos (of general KDE speediness fame) came up with some time ago and was then rewritten and integrated into libx11. Ordinarily on startup applications read input method information from /usr/share/X11/locale/<your locale>/Compose. This Compose file is quite long (>5000 lines for the en_US.UTF-8 one) and takes some time to process. libX11 can create a cache of the parsed information which is much quicker to read subsequently, but it will only re-use an existing cache in /var/cache/libx11/compose or create a new one in ~/.compose-cache if the directory already exists.
Relevant freedesktop.org bug report