You sure heavy physics calculation areas were also smooth? Try players 4, area with pillars summoning skeletons, let them summon 1-2 waves, and all 4 players will spam their attacks. This particular scenario does not involve loading assets bug, but it is just sheer min fps test. There is lot of cunfusion with min fps, people try the game, see it works ok in normal zones with max few monsters on screen, then they claim it runs smoothly. Problem is, these crucial demainding areas, there is only few of them in beta, but most likely you will see lot more of them later, when game becomes harder, density of enemies increases, everything (including our character) can use skills faster, all that is a reason for potentially lower performance.