I have been using HTTPS on XenForo for a few weeks now and it works fine, with no noticable slow downs.
Internet Explorer grumbles the most about mixing HTTP and HTTPS content, but you can disable the warning.
Firefox isn't as fussy and is working fine on my SSL secured site, although I am having to leave HTTP open for use with ForumRunner, as it doesn't support HTTPS.
To get Chrome to not show the crossed out HTTPS icon, I have had to do some template edits and phrase edits to turn any embedded HTTP:// references into HTTPS:// version, which may not work of course.
I don't use the Facebook, Twitter or google analytics featuers, so it's not a problem for me.
If you paste YouTube links (which use HTTP), it brings back the warning about mixing HTTPS and HTTP, but you get used to ignoring any warnings.
I haven't made my mind up yet about using HTTPS yet, but it's not looking too bad, I just hope ForumRunner works with HTTPS in future and I can then enforce SSL for everything.