Do you have "On Network Drives" ticked under "Scan Options" for the On-Access Scanner?
Try unticking it, and check if the speed to open is the same or not.
I don't recommend removing that option, but am curious whether it solves the issue for you.
On Network Drives isn't selected. Tried excluding .xls files also. Works ok when on-access scanner is disabled though.
I have found Excel is slow to start up the FIRST time of the day, after not being used for a long time, or after a DAT update. It's basically taking longer for any integrated scanning to load. Once done, it usually starts up and loads files more or less normally.