As a part of my normal workflow, I convert my photos to TIFs for editing using LZW compression. The TIFs were 14 mb and the JPGs were 2.2mb, but both were just as slow in iPhoto. So, I thought that perhaps the problem isn't file size, but that iPhoto took time to both decompress and scale the thumbnails and photos. So, I converted my next set of 350 pics to uncompressed TIFs and sure enough, iPhoto was much much faster, specifically organizing and scaling the thumbnails as well as laying out a book. Since you should convert your photos to TIFs for editing and printing anyway, and hard drive space is inexpensive, try uncompressed TIFs for a much faster iPhoto.
Oh, you can use iPhoto itself to convert the files by exporting them and selecting TIF if you don't have Photoshop or another scriptable application.
[robg adds: I haven't tested these claims yet, but found the concept interesting. Can anyone confirm the speed increases with uncompressed vs. compressed images?]

