printing large and file size

[ThailandTom]

I am commissioned to print 19 large images, 20 inches by 14 and the resolution is set at 400. I think this should be ok, but the problem is, when I go to work on these images my mac slows down to the pace of snail. Is my hard drive too full, or can you reduce the size of a file without losing the quality?

Any comments appreciated, tom