Slow visual calculation (PREVIOUS and RUNNINGSUM)

Background:

I'm trying to add a visual indication of groups (each person can have multiple rows) to a table.

I have made a visual calculation that performs this in three parts. 

Part one detects rows where there is a change of group:

DC = if(PREVIOUS([Person Key], 1, ROWS) = [Person Key], 0, 1)

 
Part two turns this into a group number

GN = RUNNINGSUM([DC])

Part three turns the group number into a colour using MOD of the group number. I'm leaving it out of the scope here as I get the performance problems without it and how it works is a whole other distraction.

The result is fine for tables with low numbers of rows or simple queries. Higher row counts or more complex queries can make it take a long time to complete or give a resource exceeded error. This can be in the range of a few thousand rows. It's hard to pinpoint the exact amount as sometimes the resource error only appears once the table is scrolled. Typically, the table would be filtered and reduce the row count, but I don't want to show the users a broken visual if they haven't applied filters yet - and it's possible some filter combinations will hit the limit anyway.

It's definitely the visual calculation as I can run a page with two identical tables except only one has the VC. The table without the calculations will load instantly and the table with them will lag severely. I used this method as it should be robust if the user sorts the table and never display false groups. I've tried various things to fix the calculation. Adding the MOD to the group number VC doesn't help. A calculation asking for its own previous value is counted as a circular error, so I can't skip RUNNINGSUM. RUNNINGSUM won't accept an expression so the change part has to be separate.

Is there another way to do this that performs better?