Debian may not run very fast if you have an actual install on a slower USB driver, though this is improving with faster USB 3.0 dongles. The fastest solution would be a Live session on the drive with persistence, so your changes can be saved from session to session.
A USB 3.0 external SSD or rotating hard drive should also be pretty fast and support an actual install.
The MX Linux repositories
: Backports galore! If we don't have something, just ask and we'll try--we like challenges. New packages: Inkscape 0.92.1, Pale Moon 27.2.0, Liquorix kernel 4.9-18, mpv 0.24, Kodi 17.0, Shotwell 24.5, Mesa 13.0.6