When you’re deploying DLLs into your production SharePoint environment, it’s important to deploy DLLs built in Release mode. When you do this, you ensure the code is optimized, and are reducing the risk for performance issues. There’s a nice tool on CodePlex that will help you inspect DLLs to ensure they’re built in release mode with the proper optimizations enabled. Make sure you’re looking at your DLLs and ones provided by 3rd parties or other internal development teams. I’d add this tool as a check to any code review/sign off process you currently have in place, or should have in place.