Tim is correct...it's processing time. I've noticed the same thing when using
Documents To Go on my original iPad. However if I open a basic Excel file in
Numbers the response is much faster. The downside is that Numbers cannot handle larger, more complex spread sheets. My WAG is DTG requires more processing activity to get the job done. I also have
Office2 HD and it's a bit faster loading and updating spreadsheets than DTG as well.
DTG is a good app but if all you need to do is work with a simple list (as long as there aren't complicated formulas, etc.) Numbers is probably a better bet.
If all you need to do is view a spreadsheet (not work with it)
GoodReader is a great option as well plus it's a very robust app for viewing and saving many other types of files.