It's an Apache extension that allows filtering of certain requests that you control via a rule list. I would leave it off unless you actually have security problems.
It's for cpanel security or for websites security? and it won't slow down the websites. right?
I've only used the extension directly but I imagine it's for the website. It won't slow it down but you may block some traffic is there is anything odd about it and it matches some of the filter rules.