Yikes....that sounds dis-heartening!
I too am running Mint 6 with OpenOffice 3.1 (2Gb, 1.6g duo, ibm video, Compiz advanced)
To be honest I haven't run 3.1 Spreadsheet, at all...until a second ago. I had no delay in getting a cell contents to become bold with either method, immediately.
I HAVE found Writer and Presentation to be speedier (opening, closing and normal operations), crisper looking.
I hope someone who has seen and corrected this can give you an idea? (I would try completely removing OOo and then reinstalling?)
Malachi 3:10 NRSV
...says the LORD of hosts... I will not open...windows...for you...
Linux user since 2001 pda; 2006 lappy