Hi tiguil
Could be a number of things.
How big is the file?
Is it a network file and is this newtwork known for being sluggish?
Are there any links to external workbooks in this file? Where are they linking to?
Is there a macro firing on the workbook opening that is slowing everything down?
You don't have to answer these questions, they are just a few of the many (more popular) reasons as to why a workbook may be opening slowly.
HTH
DominicB