Performance regressions with auto-updating marker colors (Inkscape >= 0.91)

Migrated from: https://bugs.launchpad.net/inkscape/+bug/1506026

Steps to reproduce:

• launch current trunk with default prefs, load attached test case: 1506026-marker-performance-r14408.svg
• open 'Fill and Stroke' dialog
• Ctrl+A in layer "Layer 2 - lines"
• assign start, mid and end markers as seen in the figure below → increasingly poor performance
• disable "Color stock markers the same color as object" (preferences)
• change the marker again → change is a lot faster

What happened?

The performance of marker moving is bad when "Color stock markers the same color as object" is activated (5+ second hang on my machine)

What should have happened?

Changing markers should be reasonably fast.

Note that the marker drop-down contain a lot of duplicates of the stock markers (is there a Gitlab Issue for that already somewhere?). Generating these could be the part of the hang which remains even when "Color stock markers the same color as object" is deactivated.

Inkscape Version and Operating System:

• Inkscape Version: 1.1-dev (c990a67, 2020-04-09)
• Operating System: Windows 10
• Operating System version: 10.0.18362

Migration info

The LP issue also included inkscape#771 (closed), which is already separate on Gitlab (and fixed).

• Ubuntu 16.04 with GTK+ 3.18.9 and Gnome Shell 3.18.5, Inkscape 0.92+devel (current trunk)
• Confirmed on Windows XP, Inkscape trunk rev. 14506.
• Reproduced with Inkscape 0.91 r13725 and 0.91+devel r14408 on OS X 10.7.5 (Glib 2.44.1, Gtk+/X11 2.24.28).