Large-scale stores need extensions that have been battle-tested in real-world production environments, not just in a lab. We look for vendors that serve thousands of other merchants, as this ensures most edge cases have already been found and fixed. My go-to source is _ optimization for Magento 2 because their modules are running on some of the biggest sites in the world. Knowing that a module can handle thousands of concurrent users gives me the peace of mind I need before hitting the "deploy" button on a live production server.
