The ACID-compliant transactional performance is intrinsic to the underlying data store, which in this case is a key value store. So I don't think OLTP (transactional) performance would be an issue. What I would question is just how versatile, extensive and SQL-compliant SQL Layer might be in its capabilities. For example, FoundationDB said it's entirely focused on OLTP use cases, NOT analytical (OLAP) use cases. So if you're application is high-scale query and analysis, this probably isn't the product for you. But then, you don't usually look to the NoSQL camp when you're addressing analytical needs. You usually go there to find a foundation for modern Web-scale and mobile apps.