ash/documentation/topics/calculations.md

taken from allow_async? This is useful for calculations that are very expensive, especially when combined with complex filters/join scenarios. By adding this, we will rerun a trimmed down version of the main query, using the primary keys for fast access. This will be done asynchronously for each calculation that has allow_async?: true.

  Keep in mind that if the calculation is used in a filter or sort, it cannot be done asynchronously,
  and *must* be done in the main query.