Before you deploy your new Pega Marketing application, establish a performance benchmark. This allows subsequent, iterative performance tests against the baseline to help identify any degradation in performance resulting from development efforts.

Configure performance test scripts that will be used for performance benchmarking.

This is well described in the previous task Identify performance testing cycles and test case criteria

Specifically to decisioning, Decision strategies in data flows have been enhanced with automatic optimization and customizable sets of output properties that improve data flow efficiency from 8.2

Automatically optimized strategies save time by reducing manual actions to a minimum because you no longer need to manually optimize every strategy component. Strategies in data flow runs achieve a performance increase with no effect on your hardware when you customize the list of output properties. You can accelerate data flow processing by limiting the number of output properties to only the most relevant.

Please find an example of this here:

https://community.pega.com/knowledgebase/articles/whats-new-pega-platform/optimize-decision-strategy-performance-82