I have been enduring this problem for a long time. I'm using a  2GHz laptop, with plenty of RAM (1 GByte) each time i startup the computer, log into GNOME and click on the applications menu present on the gnome-panel it takes several seconds to show up. During this time the hard-drive is working a lot.

It is a shame that with such powerful computer simple tasks such as opening a menu take sooo incredible long.

I don't have time to implement anything, but i think there are two options to fix this problem:

1) Whenever the gnome-panel is started, a thread could be started that would read the /usr/share/application contents such that the menu is already populated and you only have to show it when the user clicks on it. 
The problem with this approach is that it will take up some RAM.

2) Create a cache similar to that available for mime types (mimeinfo.cache) that is updated each time an application is installed or updated. Reading from only one file should be a lot faster than reading from something like a hundred desktop files (i have 118 .desktop files in /usr/share/applications).

This way such cache could be read whenever the user clicks on the menu and its contents loaded quite quickly, i think.