Vagrant box downloads are extremely slow and there are no other trusted sources (links, torrents, magnets) available. But there's an alternative way to download your Linux image from faster source and use it with your Vagrant. Instead of downloading the Vagrant box, download the Oracle Virtualbox "ova" image file then convert it to Vagrant box image.
Download the Oracle Virtualbox "ova" Linux image file. In this article, CentOS 8 "ova" Linux image file downloaded from https://www.linuxvmimages.com/images/centos-8 will be used as an example.