How are you finding bolt? I was looking into it for a personal project and the requirements that composer introduced for auto installing addons made life complicated.
Though I guess that is really a composer issue, the inability to not use something like curl when allow_url_fopen wasn't available seems insane to me. allow_url_fopen is not a great thing to have enabled in production.
I've found bolt to be completely awesome. I've made 5 or 6 sites with it, made a couple utility extensions, and used the local extension system to do stuff like custom search, a basic shopping cart, and a custom data export feature. Aside from the technology stuff it's got a positive community, good documentation, and active user base on the forum and chat.
Were there major improvements to autoloading in PHP7?
It's possible that when switching to PHP7 opcache was enabled by default, whereas it may not have been beforehand. Most likely opcache is responsible for the autoloading improvement.
That said, a faster parser and intepreter would do autoloading faster as well, so it may not be opcache at all and just by virtue that the autoloaded classes are interpreted faster.
For our sites running Bolt - a PHP CMS which makes heavy use of autoloaded classes - we saw a performance improvement between 45 and 55 percent. For Wordpress, it was around 30-40%.