Here are some things to consider when building complex business rules to make them run faster:
Use 'Jump to Step'
Multi-step calculations will perform faster if the "Jump to Step" component is used in the THEN step and the calculation is defined in the ELSE step. If the reverse is true, the business rule performs the calculation in the THEN step even though it is not relevant (i.e. outcome of the condition is not TRUE).
Table Size
When creating filters on Table Lookups consider the size of the table being referenced. If the table has more than 500,000 entries such as Data Provider tables and the filter uses a Function to retrieve a value e.g. Country Code is equal to [Function -> Get Code from Country Name], do not use the "Function" component in the filter. Instead, create a separate business rule to perform the function and when defining the Table Lookup filter, select the "Other Business Rule" component so that the result from the select business rule cascades into the logic used for filtering.