What is the actual finished size of your banners?
I often do artwork for display banners where the end size is a width/height of 2 or 3 metres.
If they include Photoshop images, I usually do them in Illustrator with a dpi of 300 and always do them at half size. (In fact most printers specifically ask for this, and don't want full-size)
I wonder if you are doing your artwork bigger than is necessary