Dark mode take more time to load the picture than light mode

I'm making my app support the dark mode, but I encountered a problem. When the app is in the dark mode, it will take much more time to load the picture than light mode. I hope to get help, thank you very much.

Please see this link for details https://stackoverflow.com/questions/59840935/dark-mode-take-more-time-to-load-the-picture-than-light-mode