Combining .mi files for speed?

I have a file with LOTS of data in it. 21 meg .mb file. It takes about 2 minutes to save out a .mi file fir a single frame. For my 450 frame animation it will take 15 hours just to write out the .mi file. YIKES! Is there a way to write out everything that's stationary for just 1 frame into one file, and write the moving stuff into another file & piece them together somehow? Or is there a way to speed up the export process.