As someone who switched from Slackware to Void several months ago, Void boots far faster. Also, though I haven't used Arch, Void really feels like a combination of a slimmed down Slackware + NetBSD (the author of Void is a NetBSD commiter IIRC). Of course, Gentoo portage was inspired by BSD ports, which I believe was an impetus for Arch's package manager.
I don't know what the story is for Arch, but I find that Void strives to make the system dead simple: it is even more minimalist than Slackware (it feels more like a BSD). It isn't just small, though: it has good defaults. There isn't a whole lot of documentation beyond the well-written man pages, and I believe this is in part because it isn't necessary.
Like Arch, though, it is a rolling release system: an update may break your system, and package indices quickly become stale as old packages are deleted from the server. You can easily fix this by updating the index.