Yup, the others have it correct. Your problem isn't because of the program that you're using, it's the size of the image. Most people don't realize that the size of the program on disk (90MB in your case) is not even close to the size of the image in memory. Lots of things contribute to file size, but the major ones are width, height, resolution, color depth, and number of layers. If the image is too big to fit into memory then you have to decrease one or more of these items until you can work with it. Either that, or get more RAM. Otherwise your machine has to page everything to disk (virtual memory) and that is what slows everything down.
As an example, my CPO image was 16.5" x 39" @ 300dpi (multiple layers) and was over 700MB when expanded into memory. It was only 50MB or so when saved to disk.
Open that file and take a look in Photoshop/Paint Shop and i'll bet you'll find that even though the file is 90MB on disk, it's 800MB or more in memory.