Tested openoffice-1.0.1, 1.0.2 and 643C under Phoebe-2, and rawhide. System is Pentium 4, 1.8GHz, with 2GB RDRAM. We are running a scientific program, which takes about 177MB of RAM and no disk i/o. We have been running this program for a long time and it had no effect on openoffice performance. Now it does. While all other applications run at normal speed openoffice windows take a minute or two two open after every click, e.g. click on the File menu, a minute later you'll see the menu. Other applications like Netscape 7.01, KDE applications, run at their normal speeds.
You may wish to run OpenOffice from the terminal so that you can see messages displayed to stderr. Also, providing a strace of the process will assist developers in seeing what is going on with your system.
A hack in the kernel has been removed and thus OOo which foolishly calls sched_yield very often is yielded as it requests. Try rawhide openoffice (e.g. 1.0.2-2).