Ghostery started using 50% of the CPU on one of my machines. Maybe the same problem. Removed it and went back to Noscript for a somewhat similar purpose. Noscript has additional benefit of freeing memory and improving performance because unwanted scripts are not even loaded.
Usually clearing out the directory where personal Firefox settings are stored fixes these problems but is too much hassle — passwords, etc.
Ghostery blocked too many things, I feel, and thereby made several webpages not even load. Perhaps it did it's job "too well?" I liked Ghostery, but if it is indeed to blame for slowing my computer down then I won't use it.