Sunday, March 31, 2013
I removed all Add-ins (File > Options > Add-Ins) and even started it in Safe Mode (Run... excel /s), but it still lagged on close.
Not surprising, Excel has a default template like Word's Normal.dot named ExcelXX.xlb, XX being the version number of Excel, in this case it's 2010 which is version 12.
Once Excel12.xlb was renamed in C:\Users\%username%\AppData\Roaming\Microsoft\Excel, the file was regenerated upon start starting Excel and from then on, it closed without hesitation.
For further troubleshooting steps, visit the overlords at Microsoft.