I havent been during FJ fest but did go during the JK fest last year. I got in early enough to get a spot in the tow rig parking but it was already a mess and it felt like I was damn near by the old office before I got past the line of rigs to get turned around.
They did spend a bunch of time in the parking lot and we only ran into traffic on the trails a couple of times but the parking lot was enough of a disaster for me. Lines to pay were a little long but they seem to go in waves so look for a lull in people.
I'm not saying avoid that weekend, just plan accordingly. Although I'm avoiding that weekend.