Nintendo appears to be using the inferior 50Hz mode for games on the European Wii U Virtual Console, whereas other regions can use the 60Hz mode.
You lot noticed the issue after downloading NES title Balloon Fight, which is available for 30p for 30 days as part of the service's special introductory offer.
PAL 50Hz versions traditionally run slower (due to old European TVs having a slower refresh rate), with compressed graphics and borders. A video comparing the difference in speed between 50Hz and 60Hz is below.
Nintendo of Europe used the 50Hz standard for games on the Wii's Virtual Console, too. This suggests that the Wii U versions come from the same code, although you'll have re-download a new version of each game to play it on the GamePad. These new versions will individually be made available over some time, Nintendo has said. Each will also incur a small additional cost.
The issue is particularly surprising because, more recently, all NES games on the 3DS Virtual Console (including Balloon Fight) were 60Hz regardless of region.
Gamers have reacted angrily to all this, flooding Wii U social network Miiverse with complaints.
"Good game, steal at 30p, but this has the PAL slowdown, so slower music and gameplay," one person said.
"This is the 21st century. Again with that 50Hz nonsense? Why must Europe still get the worst version?" another user queried.
Eurogamer has contacted Nintendo for a response - we'll update when we hear back.