Slow buffer updates with Undotree plugin

Undotree is a plugin that adds a visual representation of Vim's undo tree. When this tree view is open, certain operations (undo, redo, joining lines) are slow in the GUI, but not in the terminal.

To reproduce:

1. Add the Undotree plugin to ~/.vim/pack/mbbill/opt/undotree so that it will be found by packadd:

mkdir -p ~/.vim/pack/mbbill/opt
git clone https://github.com/mbbill/undotree ~/.vim/pack/mbbill/opt

2. Start MacVim with mvim -u vimrc where vimrc is

set nocompatible
set undofile
packadd undotree

3. Open the undo tree visualiser added by Undotree: :UndotreeToggle
4. Insert 100 lines: 100ii<CR><Esc>
5. Go the first line: gg
6. Join all the lines: Hold J

There should be noticeable lag. Now repeat steps 1-6 except with mvim -v -u vimrc in step 2. Joining the lines should be quite snappy.

I've attached two videos below showing the behaviour in each case, and I've included my system details.

GUI:

https://github.com/macvim-dev/macvim/assets/123784102/4e1828c4-4e40-4f29-9171-6c77a1b15f9e

Terminal.app:

https://github.com/macvim-dev/macvim/assets/123784102/26cc3bda-fd04-4165-9bd1-712da0132d45

System details:
OS: macOS Ventura 13.4 arm64
Shell: zsh 5.9
Terminal: Terminal.app
$TERM: xterm-256color
Vim: Downloaded from GitHub Releases

Reproduce the lag using MacVim with mvim -u vimrc, packadd undotree, and :UndotreeToggle, then compare it with mvim -v -u vimrc as described. Trace the GUI path involved during undo, redo, or joining lines; the issue is done when these operations are no longer noticeably slower with the Undotree view open.