An SLA is only useful if it guarantees a certain level of quality. Current Cloud SLAs cover availability but ignore a key ingredient: Response and Throughput Performance. A Performance SLA would need to relate to the applications performance itself, something that no Cloud Provider has control over. We will discuss how Application Performance Monitoring can be used to define, measure and enforce a usable SLA for both sides. We will talk about the differences between IaaS and PaaS cloud providers concerning such an SLA. We will also show how this will lead to better User Experience with less R&D effort. Finally it enables us to easily compare cloud performance across vendors in terms that really matter: Response Time per Cost.