Enabling 'Use Dithering' causes drop in performance (png)

Summary:

Gradient Dithering (enabled by default in 1.2-beta) causes a significant drop in performance handling files with high-resolution images (eg. The Starry Night) (There might be a performance drop with vectors but none that I can tell). Testing with an AppImage.

Steps to reproduce:

• open Inkscape (Appimage build)
• Import The Starry Night (or drag and drop it from the file manager)
• To prevent any flickering change rendering developer settings (For instance, #6819 (comment 913678234) removes most of the flickering for me)
• Toggle 'Use Dithering' and observe performance by translating objects)

What happened?

Enabling 'Use Dithering' causes significant performance drop translating objects.

What should have happened?

Less significant drop in performance.

Version info

Inkscape 1.3-dev (09700cf, 2022-04-14)

    GLib version:     2.64.6
    GTK version:      3.24.20
    glibmm version:   2.64.2
    gtkmm version:    3.24.2
    libxml2 version:  2.9.10
    libxslt version:  1.1.34
    Cairo version:    1.17.6
    Pango version:    1.44.7
    HarfBuzz version: 2.6.4

    OS version:       Ubuntu Jammy Jellyfish (development branch)