I always feel somewhat bad when I do this, but it really is much easier to read the first few comments. For really long articles, I often only read the beginning, catch something I want to comment on, and hope I didn't need the rest for my comment to be valuable.

It's not great, and honestly feels a little like in high school when you didn't actually do the full reading assignment for english class, but it can be hard to keep yourself to high standards.