What calculation? You mean the Derivative math required to determine how many people are around and change % damage and effect on them?
uhm, not really. A desktop with a quad-core and 32gb of ddr3 could handle that.
Its the actual rendering of individual particle effects at a massive rate which is causing the issue. It has to be. Their network engineers must agree because they are limiting particle effects as well as individual tracked assets (see bugs and deer).
Making the game pretty is causing the lag.
If they forced perspective at 800x600 30fps at low settings, you would be able to have > 1,000 people in the game. They are sacrificing quantity for quality.