I would guess MediaLab after some amount of time (I don't know how much) decides an outside program (like Word) is not working properly, and moves on. This would explain why starting the program in advance is helping.

However, why Word would be opening slowly could have many potential causes. What are your system specs, and how do they compare to the specs for Word 2010, MediaLab (which version?) and Windows 7? How many programs do you have running at once, and do you have any large background programs (e.g., some anti-virus programs)? Is the Word document especially large, perhaps because of embedded images or other media?

I think your first steps are to check the specs, and if it seems like your system should have no trouble doing what you're asking it to do it, give the computer a basic check-up. Make sure updates are installed, scan for malware, etc.

Let me know if that helps.