1. start emacs with “emacs -Q”
2. eval the following form:
(progn
(setq ddd-tm (current-time))
(switch-to-buffer-other-window "*Messages*")
(message "ddd: %ss" (float-time (time-subtract (current-time) ddd-tm))))
The result is lager than 0.1s on my machine. This makes undo-tree running too slowly, because it calls switch-to-buffer-other-window twice per undo.
The emacs 25 has no such problem.
OS: Centos 7.4 x86_64
Emacs: 26.0.91
GTK version: 2.24.31