set expression taking ages - sometimes crashing.

[#1]

avg(aggr(Count({$<questionGroup={'D1Grid'},Data={'*(r2)*', '*(r3)*', '*(r5)*', '*(r6)*'}

  ,potentialResponses={'*(r28)*',

  '*(r29)*',

  '*(r2)*',

  '*(r6)*'}>}Data)

  , ID, Data))

Any ideas why something so simple would crash?

The document has a few million rows loaded into it.

[Gysbert_Wassenaar]

You may find it 'so simple' but it requires quite a bit of calculations. And probably memory too. Open the Sheet properties window, go to the Objects tab and look at the CalcTime and Memory columns. If you run out of physical ram memory then Windows will start using the page file and that slow things down horribly. Running out of memory can also cause a process to get killed.