I'm going to assume that you're using Sub-D surfaces even though you don't make mention of it in your post. Anytime I'm working with Sub-Ds I find it MUCH easier to convert to polygons, texture it that way, and then convert back to Sub-Ds. If you're still lagging in the UV editor may just want to check exactly how many UVs are being mapped at once. If your dog has 16,000 faces or something it'd probobly slow you down.