When selecting "View - Explore changesets on this branch", sometimes the message "This branch has a large number of changesets ... Continue?" pops up AFTER a lengthy calculation and if "Yes" is pressed, the "Branch Changes" window is then displayed instantly! ;)

Taking a hint from TortoiseSVN, you could load only the latest 100/1000 changesets or so, and then have buttons to "load 100 more" and/or "load all"...