Yeah nationals are huge. Especially out there. And yeah it does take forever to get through all the motos, as there's always at least 80 on Saturday and Sunday. The only thing you really need to know is people are a lot more aggressive than other races. Nationals are a lot of fun though.
When my kids were racing 8-10 years ago BMX was at its biggest with most West Coast Nationals 300-350 motos and the Grands was over 600. When we raced the southwest nationals it was in Desoto, Texas and was about 200 motos. It was pretty normal for a friday night local race to have 30-40 motos.