Slow visual calculation (PREVIOUS and RUNNINGSUM)

You need to avoid a RUNNINGSUM as that is an inherently expensive operation. If what you are trying to achieve is to visually highlight when rows belong to the same or different people, I think I would assign each person a colour, so that no matter what the order of the visual is the same person always has the same colour.

I think you could do this by using the RANK method I proposed and applying the MOD logic within the measure, or using some other way to turn a unique Person Key into a unique colour.