Getting slower: most operations take about one minute.

Hi all,

I am editing a spreadsheet using Numbers 2.3 on an early 2011 MacBook Pro. The spreadsheet has four sheets, each about 1,200 rows and about 14 columns. Each sheet has some references to one other sheet. Most operations take about one minute to complete, while some things take only a second or two. During the lengthy operations, Numbers displays the spinning beach ball, CPU usage on one CPU core goes to 100% and Activity Monitor says that Numbers is "Not Responding". After the minute goes by, all is well, so it didn't hang, it's just busy. Even just changing the text of a header cell shows the same behavior. This seems far longer than it took in weeks past - it seems like something changed that made everything work much more slowly.

Does anyone know if this reflects a problem with the spreadsheet itself and, if so, what can be done about it?

Thanks,

Scott