My experience is that LMDE Xfce is faster, but only slgihtly. The problem (if you want something that's light) is that Mint Xfce editions have lots of heavier stuff installed by default. I find that Ubuntu hits a wall around 150 MB of RAM - it doesn't go too far below that easily (although you can do it, especially if you scrap Xfce and use a WM). I did a test to look at a Debian net install with no desktop environment or other non-critical packages installed. I then added the mint repo, the multimedia repo, and the contrib and non-free repos. I did an apt-get update, apt-get install linuxmint-keyring deb-multimedia-keyring, apt-get update, apt-get dist-upgrade, apt-get install lightdm xfce4 xfce4-goodies lxtask htop. The latter 3 are optional. I found that on restart after login I was using about 74 MB of RAM, which is well under a lot of other desktops/distros. I think you could get something fairly similar with CrunchBang (the testing version) with xfce. The point of this is that the Mint Xfces won't be as lightweight because they have lots of gnome packages.
Dell XPS 15 l502x - Debian Testing 64-bit NetInst Xfce, SolydX 64-bit Debian Testing, SolydK 64-bit SolydXK Testing
Old Gateway Pentium 4 Desktop - Arch Linux 64-bit Xfce and SolydX 32-bit Sid