Have you upgraded to the newest version of google spreadsheets? We were having some performance issues that sound just like what you're experiencing and as soon as we made the switch, our sheets were super fast.
I've tried copying a problem sheet to a new file and first impressions are positive. Thanks.
"All spreadsheet limits mentioned above have been removed in the new version of Google Sheets. The new version of Google Sheets should support 2 million cells of data, though please note that extremely large spreadsheets may have slower performance. Learn more about switching to the new version of Google Sheets."