That's a strange reply, as in the Branches view (List view mode) all the branch comments are downloaded at once and that does not seem to be a problem! Also, when issue tracker is activated, all the issue-tracked branches will have their branch comment displayed in the tree!

I'm thinking you may have misunderstood me? I'm talking about branch-comments, not changeset-comments! (There is only one such comment per branch and that can surely not be a performance hit compared to fetching all the changesets that are rendered? We seldom write novels for branch-comments anyway... ;)

Comments are not actually downloaded to render the BrEx for performance reasons. They are only downloaded on demand when you click on a branch. This is done this way to avoid potentially downloading (and extracting) tons of data for nothing.