Google Analytics I assume ? check the browser version and profile of your visitors. Sometimes alot of your traffic are bots using old browsers which may not support your server's peferred SSL ciphers so can't visit the site.
My Centmin Mod forum's premium members get acces to custom GA Dashboards I create including ones for browser breakdowns https://community.centminmod.com/posts/25254/. Very handy to see what makes up your real traffic etc. But you can create your own GA custom Dashboards too.
So if the majority of lost traffic is to WinXP IE 8 browser sessions durations of 1-10 seconds, you know you only lost bot/crawler traffic with switch to HTTPS etc
I read that Xenforo will automatically detect if you're using http or https and use the proper version of analytics. What I'm wondering is should I be changing the default property URL in GA to the https version? How will this affect my GA data?