Factor's graphical user interface library, written completely in Factor, could be faster if certain changes were made.
Factor's recently-added support for typed words, specialized arrays, and SIMD could speed up geometry and layout calculations.
Making better use of modern OpenGL could improve rendering performance. All vertex and texture co-ordinates for a single window could be combined into a single vertex buffer, instead of creating vertex arrays and switching them for each UI element. All textures for UI elements could be combined into a single texture atlas, instead of switching state for each element.