Super Mario Bros. - NES (1987) - 50 Hz
The NES version I own is a completely unoptimised version. The gameplay is slower, the animations are slower, the music is slower, the aspect ratio is slightly squashed and the frame-rate is slower. It feels very noticeably slow to play.
Evidence: Compared PAL NES, PAL NES with Duck Hunt, PAL GBC Deluxe, PAL Wii Virtual Console, PAL 3DS NES Virtual Console, PAL 3DS Deluxe, & PAL Wii U Virtual Console.
Super Mario Bros. / Duck Hunt - NES (1988) - 50 Hz with various optimisation
The NES version with Duck Hunt that I own has been changed a lot from the earlier NES version I tested. The gameplay has been optimised to be more or less the same as the original 60 Hz version, however the music is now way faster than it should be. Additionally, the animations still run at the slower 50 Hz speed, the aspect ratio is still slightly squashed and the frame-rate is of course still slower. They didn't update the demo that plays if you wait on the title screen though, so with the new gameplay speeds Mario follows a different path.
This strange combination of slow, correct and too fast makes for a very odd version to play, and I found it always feels a bit too fast, even when I muted the far-too-fast music. I could never find a test example to prove it, but I felt like the gameplay might be slightly too fast as well. The in-game timer runs at exactly the same speed as the 60 Hz version though, so my feeling that it's too fast may just be caused by the odd juxtaposition of the faster gameplay with the slower animations, it's hard to say.
Evidence: Compared PAL NES, PAL NES with Duck Hunt, PAL GBC Deluxe, PAL Wii Virtual Console, PAL 3DS NES Virtual Console, PAL 3DS Deluxe, & PAL Wii U Virtual Console.