Sonic is supposed to be fast. In fact, as an otherwise nebulous, frequently-rebooted character, that's really all that can be said about him for sure... unless you grew up in Australia, where that was never the case to begin with. In early games Sonic's land speed was markedly slower abroad, and it's all due to differences between PAL and NTSC. Don't get too worried about the acronyms, all you need to know is that they're two different ways of encoding colour information for analogue TVs. NTSC was mainly used in the Americas, Japan and South Korea, while Australia, much of Europe, Africa and chunks of South America used PAL.
So what? Well, these systems run at different frequencies, which meant that the same game running at 30 frames-per-second in the US would output at 25 in the UK. Hence, one slow hedgehog.
Of course, this is an oversimplification. Many games were PAL-optimised to run normally — the first Sonic the Hedgehog just wasn't one of them. Also, thanks to digital TV, almost none of this stuff matters any more. Whether or not there's been a good Sonic game since the advent of digital TV, well, I'll leave that to the comments section to decide.