The old Gmail is dead. Slow and ugly are the future. If you want something clean and fast, you're going to have to switch to something else. That's the magic of SaaS.
I've noticed the "return to the old interface" option is gone too, but it was never likely to be a permanent option, and I'm not surprised it was removed.
"Google has transitioned Gmail accounts to the updated version that it released last spring, and is now removing the option to go back to the “classic Gmail.” (The stripped-down HTML view is still available for some browsers but lacks many features)."
If you or your organization uses Google Apps, then for now you aren't yet forced to switch. At least that's been my experience. I'm not sure whether the rollout just hasn't made it to my domain yet, or an actual setting is controlling it.
I've noticed the "return to the old interface" option is gone too, but it was never likely to be a permanent option, and I'm not surprised it was removed.