Every time I start LibreOffice, I have to wait 3 minutes before I'm able to start working (screen is grey). Why? Are there any ways to fix this problem?
How you can see here LibreOffice won't work well with ALL Java versions. The 1.6.0_21 seems to be the best java to use with LibreOffice. The 1.6.0_20 & 1.6.0_22 seem to be in a fairly good 2nd place place. The 1.6.0_24 was really bad and the 1.6.0_26 was even worse apparently.
To install 1.6.0_21 You should first purge every other Java versions from your system (icedtea, open, ecc. all included). Unfortunately, it seems near impossible to find compiled packages for precise, so you'll have to consider to build it from source (https://launchpad.net/ubuntu/+source/sun-java6/6.21-1ubuntu1).
Otherwise, you could disable java plugin for LO, from Tools menu, then Options, Java, uncheck the box.
Last alternative, install other versions of java (such as the last Oracle 1.7) and select it in the list, to see if the problem solves.