Couple of reasons why Excel can take a long time to save.
Biggest reason is how many formulas you have in the spreadsheet. One of the most common practice we all have a habit of is to put the formulas in EVERY cell of EVERY Row listed. If you have a few thousand numbers in your sheet, and each row has 3 or more cells with formulas, it can get very tedious for Excel to close out a file. If you noticed in my spreadsheets, the sheets with the lists of numbers ONLY had 1 row with the actuall formulas. The rest of the rows were set to have the data only, making it easier for Excel to shut down. Excel is not having to calculate anything before shutting down.
Another issue is, you could have external references. Click Edit> and look to see if LINKS is highlighted. If it is, you are referencing a file outside of your spreadsheet, and Excel is trying really hard to update the external file before shutting down. You must kill that link (assuming you do not need it).
Finally, when is the last time you did basic housekeeping on your hard drive. I always suggest once a week defrag and cleaning of a hard drive to keep optimum performance of the overall system.
Just a few ideas, hope they help.
Sir Metro