The “Convert to number” function in Excel 2013 takes much more time to convert data to numbers. You won’t feel it when you just convert 300 cells, but if you have 6000 cells i.e., you see a clear difference. While it takes just 1 second with Office 2010, it takes 13 seconds in Office 2013 having 6640 cells converted.
I opened a Microsoft support call, Microsoft is now aware of that bug, but doesn’t want to provide a hotfix for it (request has been declined twice). The workaround they provided in my case takes more time than just waiting…hopefully it’s fixed in a newer version.