The approaches you take to ensure optimal performance with SharePoint is different in the new SharePoint Add-in model than it was with Full Trust Code. In a typical Full Trust Code (FTC) / Farm Solution scenario most code operations took place in the SharePoint Server-side Object Model code.
In an SharePoint Add-in model scenario, the SharePoint Client-side Object Model (CSOM) and/or the SharePoint REST API are used to execute code.
The major difference between the two models is server-side code versus client-side code. In the SharePoint Add-in model, because code is executed via clients, additional network traffic occurs. Minimizing the number of API call round trips to the SharePoint server will increase the performance of your SharePoint Add-ins and reduce the amount of resources consumed by your SharePoint site.
Additionally, in the SharePoint Add-in model, because code is executed via clients there could be long delays before a response is received. Caching data for long running operations (like the User Profile APIs) can reduce the amount of time it takes to return information or receive confirmation a process is complete.
You can read the rest of this Office 365 Patterns and Practices Guidance article on GitHub, here: https://github.com/OfficeDev/PnP-Guidance/blob/master/articles/SharePoint-Add-In-Recipe-performance-considerations.md