It might just be the amount of players constrained in one area all doing skills at once. I felt the full force when I dove in with magical sparks everywhere and a sudden delay of 2 seconds later AOE skills rained thunder with bodies everywhere. The delay is not really lag or anything, but the amount of players on the screen at once.
Janus is fine as it is, for a 35 teams. of 70 pop. Turnpike may have trouble because that is what hit me hardest with 80 player teams and having about 120 people meet at the center on screen is beyond surprising that the instance is still running nominal. People have to spread out to lessen the concentration of the center, I see almost no one take the side routes or have side confrontations.