Slow macros on Excel 2013

Hi,

I have a workbook containing a system of macros that I have been developing for several years. The macros work well on Excel 2003 and Excel 2010. On Excel 2013, on the other hand, they sometimes work ok, and sometimes work unreasonably slowly.
I don't intend to bother anyone with my code because it's very complex. I do have one question:
I noticed that when I open the task manager during run (without doing anything inside the task manager), it often "releases" the stuck macro and causes it to run properly.
Does this give anyone an idea why my macros work slowly on Excel 2013 and how they can be improved?

Thanks,

Chen

Thanks for the response.
The code does not alter the protection status of sheets.
I do believe that there is some modification specific to Excel 2013 that causes the slow-down.
I hope to find out what it is, and maybe find a way to bypass it.