Relatively slow. But that just means you shouldn't call it too many times in a single frame. If you only need to check line of sight for one character (or only a couple at most) then you don't have any problem. For lots of characters don't check line of sight for all of them in the same frame. Instead stagger the checks; only check line of sight for a couple characters every frame. Assuming you have a high enough framerate (ie. playable) nobody will notice that the checks are staggered.
To put it in perspective, I check line of sight using LinePick (essentially the same as EntityPick) four times per frame and there's little framerate hit.
To put it in perspective, I check line of sight using LinePick (essentially the same as EntityPick) four times per frame and there's little framerate hit.