I have had the same thing happen (sort of). In virtual box this happened out of the blue. MGSE (with 3d acceleration, etc) was fairly fast, and then it slowed down one day. Just gnome-shell/MGSE, not gnome classic or MATE. I have no idea why - IIRC I didn't install anything (besides updates maybe?). I decided to reinstall, but it didn't help. It was still slow even with just the default installed. The only difference was that the first install was from the RC. I have had the same thing happen on real hardware, too. I didn't have as much of a slowdown, but it was noticeably slower. And I have a 2.3 GHz Core 2 Quad processor, 8 GB of RAM, and an nvidia geforce gt120 graphics card (I had the proprietary drivers installed). I wouldn't say that it was horribly slow, but I don't think that it should have been slow at all.
@christoph23 Cinnamon doesn't currently ship with Mint because it didn't exist when Mint 12 was released and it is still only an alpha.
Edit: Although cinnamon is in alpha, that is more feature wise. I have used it fairly extensively and I have found almost no bugs (a lot less than MGSE). I also find it faster.
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