Building systems in the Cloud efficiently means to be smart and use the building blocks offered. One literally have everything, e.g. in Azure, to rebuild your on-premise data center. However it would not be a winner to do this….
Instead, be a fast mover and leverage provided building blocks to your advantage. Identity Management is a good example, you can roll your own in your solution or turn AAD B2C offering great value at limited costs (https://azure.microsoft.com/de-de/services/active-directory-b2c/). And talking about speed, just grab it and use it, nothing can be developed faster.