As title said, I had problems with image previews being too slow to load on my phone.
But when I loged in to the server via browser on my phone the image previews load much faster.
So then I tested on a family members iPad, and it was too quite a lot faster to load previews than the Android app.
I have tried to test and compare it on both same network as Nextcloud is in and out on the mobile net and it is always the same, android is slower to load image previews than web browser and iOS app.
(Server is running 20.0.5 and android is running whatever the latest it updated to via play store)