I haven't run any performance tests, but I saw at least a 10 fps increase when I dumped Unity's GUI implementation and switched to NGUI. FWIW, NGUI is also faster and more stable (and useable) than UIToolkit which I also tried out. I think it really comes down to how well you organize your UI. Grouping elements that are dynamic together, and static elements is a separate group (panel) will be your best bet. Also, experiment with the depth pass feature of the panel.