Key Metrics for Evaluating a New DSP

Hello everyone! I'm planning to run a trial campaign with a new DSP and I'm curious about which KPIs are best for assessing the vendor's effectiveness. While Click-Through Rate (CTR), Cost Per Click (CPC), and Impression Share seem like the obvious choices, I'm eager to hear what metrics others find useful. My goal is to evaluate the vendor's performance rather than the campaign's outcome specifically. Any insights or suggestions would be greatly appreciated. Thanks in advance for your feedback!