One worksheet causes severe "not responding" delay on saving/scrolling

hi,

I'm using Excel/Office 2016 on windows 10.

i have a workbook (xlsx, approx 4Mb) with multiple sheets.  Saving it causes the Excel window to go "white" with a "not responding" for 20-30 secs.   Scrolling is similarly delayed and it makes the spreadsheet almost unusable.

i have isolated that there is one particular sheet that is causing the issue because if i delete that sheet, it saves instantly.

if i try to copy the sheet to another sheet or an empty workbook, Excel hangs completely.

if i select all the cells and try to copy/paste them as formulae to a blank workbook, Excel hangs completely.

if i select all the cells and try to copy/paste them as values to a blank workbook, that works.

i have tried "safe mode" and disabling all addins, but it makes no difference.

i have removed all external file references (by searching for '\'), but that made no difference.

i have removed all the conditional formatting but that made no difference. 

i have changed all the VLOOKUPs that use other sheets to use a specific cell range (e.g. changed 'Customers'!A:G to 'Customers'!$A$1:$G$999) but that made no difference.

what else might be causing the problem?  is there a "logging mode" or any other tool which would help to indicate where the hangup is occurring?

(unfortunately i cant attach the file because it contains company sensitive financial information.)

TIA

K