I used it in the past. You definitely feel it but it's mostly hmpa which has a weird lag i can't really explain. Wd just likes to use cpu and drive when something happens which you can notice on slower systems. On new systems you can't notice wd in most cases.
Sorry for disturbing your topic btw i will stop now.
To be honest and sorry for the late reply. Got a little busy here at work. I don't want to seem like I'm boasting in anyway about the software but i have not felt any real drag on startup or browsing. But everyone's system is different in some way.