There are lanes allocated based on pace ( in pool 2, anyhow ) however, it's the number of people in the lane that creates the problems more often.
While I agree with the idea of standing up for yourself, in this case, I would have to knock the fucker out to get any satisfaction. This is my neighbourhood pool and gym facility, and a large proportion of the patrons are not native English speaking people. I have seen on multiple occassions, people pulling the 'no English' trick on the lifeguards, and carrying on with whatever horseshit they are doing. I have also lost it several times at this pool for various justified reasons, and somehow, I always wind up looking like the asshole.
I think next time, I'll take your advice and go 'all medieval' on the guy, and perhaps the word will get around, not to mess with the 'crazy sloth'