The middle security checkpoint (near gate 12) also has a Clear entrance, and that has always been empty every time I've been there. The Precheck line might be empty, or it might be longer than the regular line -- I've seen both types of situations, on multiple occasions.
The West checkpoint (near gate 27 and above) is one I'm not as familiar with. I don't know if they have a Clear line or a Precheck line. We almost never go through there.
The middle checkpoint has Clear, Precheck, and regular. There's no fourth line -- if you're in the Clear line, they just take you to the head of whichever other line you're going through.
i.e. Clear lets you jump straight to the security part. Many airports will have a dedicated Precheck+Clear lane, which would typically be faster than either Precheck or Clear on their own.