Hi 
I guess you mean LibreOffice rather than OpenOffice?
Before opening the file try changing the "memory" settings in LibreOffice
Tools - Options - Advanced (or might be "Memory" straight-away)
Boost all these by quite a lot. If you have over 1Gb of Ram then you could easily boost them so that files can have lots of images over 10-15Mb each. The file might open faster now.
As people keep pointing out the formats that end in X are MS proprietary formats and change a bit with each release of MS Office. it's far better to get people to use the older formats that don't have an X on the end or FAR better is to use Ods. Have you tried opening the file in a different version of MS Office? if it was fast in MSO 2007 is it still fast in MSO 2010?
The file probably contains tons of heavy images or something. We can have a sneaky look without even opening the file! :) Without opening the XlsX can you just
1. create a copy of it
2. rename the copy so instead of being .XlsX it's .Zip
3. double-click the zip-file to open it
You will see various folders and one should be "images", "photos", "pictures" or something like that. There's only about 4 folders so you could have a look in each one quite quickly. When you find the images you can probably see their sizes. Are any close to 1Mb or over? I am not sure but i think you can drag them out of the zip-file's folder and then use Gimp to scale them and then put them back in top replace the heavier versions. If it works the copy of the file should now open faster.
On the other hand it might be the file contains some bad macros trying to connect with things. I don't know much about macros or if they would slow the thing down but malware has often been spread by MS macros or the macros themselves have been malware and just opening the file has infected machines. Many places block macros nowadays. LibreOffice uses different languages for it's macros and so far i've not heard of any containing malware.
Good luck with this! Hopefully someone else here might be more helpful than me!
Regards from
Tom 