Even though some messaging apps already compress images to save data, some don’t and users don’t have control over this. This app would be activated via action extension after selecting image in Photos or elsewhere and without further user input compressed the input image with maybe three levels of compression allowing users to quickly toggle between them, check quality and then via share sheet send this image via messaging app.
Users with slower connection or low data plan would save data and the sending would be quicker.
With current iPhone CPUs it should be no problem to compress image into three compressed ones simultaneously..
Note: I found a few somewhat similar apps in the AppStore but they were pretty old and not very appealing.