The file size reflects how much the data can be compressed, by taking advantage of repeating patterns (for TIF) or by throwing out awkward details (for JPG). The data is completely uncompressed when loaded into memory for editing, so it will be width(pixels) * height(pixels) * 3 bytes regardless of format. JPG might be marginally faster to load and save, since disk access is much slower thatn RAM, but actual computation speed doesn't change (and until you save it doesn't have a format, so render speed couldn't be affected - unless Poser is clairvoyant!)