Winterboard used to be much, much worse on older iPhones, and could really slow things down. I think a lot of people still have that in their minds. Of course, it still uses memory, but it's rarely doing so much that it's going to be really noticeable--unless you're using it to run a million different themes, some of them probably covering the same areas, some poorly coded or not updated for the latest firmware, some with unnecessarily large graphics, etc.
In other words, if things are getting bogged down, it's probably not Winterboard, but what you're having Winterboard run. It's similar to hearing people complain that jailbreaking slows down their phone, only to find out they have 863 tweaks and 56 themes all running at once.