It is indeed hard to optimize it; I haven't fully utilized the potential myself. For now I have an object pool which allows me to create only one instance of a canvas element. Kotlin also allows me to use coroutines easily, also (potentially) have access to GPU/CPU since this is a desktop app.

Another thing I tried (still in backlog) is dynamic quality reducer for images and PDFs - sort of like a game engine rendering where zooming out will reduce quality of images. What are you writing your app on? Electron?