As a general answer, the more powerful your machine, the faster it will open. I inherited a lovely set of 30+MB files. I had to use an equally lovely, PII 333MHz, 32MB RAM laptop to open them with. It easily took 15 minutes to open up and 5 minutes to register a change. Eventually I got upgraded to a PII 600MHz, 64MB RAM desktop. It only took 3 minutes to open up. This is not a good solution.
A better suggestion would be to make sure that Calculation is set to "Manual" (Tools|Options - Calculation Tab, Check the "Manual" box)
I'm sure other people have some more ideas as well though.