Optimizing a GUI element by using the nine slice scaling mode causes the menu to lag a lot, especially when the specific element (in the screenshot's case the dark background part of the slider) shows up multiple times in the menu. 

Below is a GPU usage diagram provided by EwanHowell5195

Purple is the main menu.

Yellow/Green is the options menu.

Red is the video settings menu, where multiple sliders show up.

The video attached shows the drastic decrease in performance whenever I enter the game menu.