The custom app parts in SharePoint add-ins (SharePoint hosted) gives bad performance when adding them into a custom page in the hosted site. Is there a solution for this problem?
The slow performance is most likely caused by the way the app parts are placed on the SharePoint page - each app part is contained within it's own iFrame which points to the app web. When the page loads, another full HTTP call has to be performed for the content from each app part placed on the page. Unfortunately - this behaviour is a part of the Add-in model and cannot be changed. Source: How to add SharePoint-hosted add-in as an app part
It has it's positives, as it allows to display content from provider-hosted apps in a web-part like manner. The biggest downside is - it treats SharePoint-hosted apps the same way.
To boost the performance I would suggest (if possible) switching to the SharePoint Framework, which is client-side only, doesn't use internal app webs and renders directly inside the page's DOM without being wrapped with any iFrames. Take a look here: Build your first SharePoint client-side web part