Distribute the calculation of alternative scenarios

Reduces the response time by a reasonable margin. Locally I measured a basic sample from ~2.2s to ~1.8. This case had 2 alternatives, so this will be bigger if there are more alternatives. For example, with HEPA on, and mech ventilation, the report went from 3.4s to 2.4s.

There is more to squeeze out of this calculation, but this is already a good improvement worth banking.