The upload of the photo is not something that is dependent on the application and\or whether it’s native or not, as you are uploading your file over to AWS S3 for storage in the cloud.
So main factors here are your internet bandwidth and the logic of how you built the UX for the upload.
As per your other issue - it is a OS dependent locale issue - Android\Desktop\iOS depending on the locale settings of the devices may use “,” or “.” for decimal separator.
You need to work on the format that is universal for all OSes, this is something a bit tricky, but manageable.
If you won’t be able to figure this out for your app logic, we have a bit of a tricky solution ,but depending on your amount of symbols after the comma (e.g. 2), behind the scenes we remove the comma\dot and then divide by the value correspondent to the amount of symbols (e.g. by 100 in our case). So irrelevant of which OS you use - your device receives and calculates the number in the correct format and displays it to the user.