[Business Rule] Adding IF statement to rule causing significant performance degradation

Summary:

I am trying to add an IF statement to handle cross-year Actual & Forecast situations. However, adding the IF statement causes the rule to increase in run time from 3 minutes to around 40 minutes.

Basically, I need the rule to calculate differently if the current Actual period is Dec of the prior year. All other situations are calculated another way.

See below for the code (note that I use Jan for the IF here because the calculation is FIXed on the forecast month, not actual month. Any recommendations on how to handle this situation efficiently?

Note: Period is dense, and as I understand it, dense dimension members are best used in IF statements as opposed to sparse