I have always noticed this on my PC. I just listened to the Raspberry Pi podcast episode 1 where Wimpy explained that this affected both PCs and RPis. The reason is because Libreoffice has hi resolution icons in the menu and has to load and render them each time the menu is activated for the first time.
Is this still the case? Wimpy said he was going to fix it (by using lo res icons?)
Thanks,
D.