Optimizing my app, need help

Hello! I am working on a fairly large and ambitious project for which I need help optimizing user information delivery and usability.
The problems I’ve had so far are:
1.- very slow menu bar to change screens; sometimes it takes up to 2 minutes to switch to the screen I want
2.- the same thing happens when clicking on buttons that direct the user to other screens
3.- loading images; I am currently working on lowering the size of the images and I have obtained good results. But I would like to know if they have specific sizes for icons and banners
4.- finally, improve the display of lists and related elements between databases