Hi Glider!
Every time I advance adding more functionalities to my apps, they begin to slow down even without having a lot of data.
I try to use “DATA” before Gsheet.
The new “Glide Sheet” functionality may solve that problem.
But, from your experience, what else would you advise me to have faster apps?
The biggest point here is to determine what causes your app to load slowly, is it because of the images’ size or anything else?
I really don’t think it’s the images
I have used image compressor, also with images on external servers, even so in visibility and filtering conditions the apps take time. 
Can you share your app with us so we can try to have a look? And what are the specific functions that load slowly?
If you can give us more info about your APP we can narrow the problem source.
Sometimes a good design helps any APP’s performance. For example: not always a Brute-force search is enough, if you can create an index field in your database/table, the performance will increase a lot with low effort.
Is the amount of pictures suspected (or the size)? Create a copy of your APP using now the same image link to everything and test the performance again. This will discard or not any problem associated to the imagine links as your 1st step to fix the issue.
Saludos @Jorge_Valdez