Actually, there are many reasons that an app/a flow can be slow to load, slow to respond, and slow to write to data sources.
The following is common issues cause an app/a flow to slow to load, slow to respond, and slow to write to data sources:
1. The app has a large number of controls/objects that directly reference on-line data 2. 2. sources for many of their properties.
3. Shaping expressions (i.e. Filter or Search) that contain clauses that require frequent
4. Shaping expressions that are not delegated
5. Use of inherently slow data sources such as Excel on OneDrive