Microsoft Office is slow in OSX period.
Even with 4GB of ram and an SSD it's still extremely sluggish. It's sad when Photoshop CS5 opens faster than Word. I installed the 2010 Office Beta last week. It appears to have slightly improved speed, though it's not dramatic.
If you can, just stick with iWork. When I used my old MBP in college I always wrote my papers in Pages and just saved it as a word document before submitting them. My professors never had an issue opening them on their PCs.
However, if you need to spend a lot of time in Excel then I'm not sure what your options are to improve speed, aside from installing Windows in Bootcamp and just using Office on the Windows partition.
I am running the beta of Office 2011 and wow. It is insanely faster than previous versions. Even though I am on an ssd drive I noticed a complete 50% increase in overall responsiveness and cold start times.
It's good. Has some issues, but it's a beta. Other than the issues, its awesome. Love the new ribbon too. Just cold started Word, and was at a workable blank document in under 1 second.
11 Will be a force to be reckoned with. Thanks microsoft!