The PAL 50/60hz guide!

For many years, basically until the introduction of HD monitors, Europe and Australia have suffered from the concept of PAL video signals.
On paper, this is not necessarily a bad thing. While a PAL video signal has a slower refresh rate of 50hz compared to the 60hz of NTSC (the standard used in the rest of the world), the graphics resolution is also a bit higher, allowing for a slightly better image quality.

The big issue however exists primarily in the field of video game consoles. Even though some games were developed in Europe, the hardware and development kits would usually come from Japan or North America, where NTSC was the standard. At the time (and basically up until recent times), all console games would be timed based on the framerate of the game, rather than the internal clock of the computer, as you would commonly see on modern hardware. This is also why older console games tend to run slower when the framerate drops, as opposed to merely skipping frames. The entire game logic is closely tied to the refresh rate of the graphics.
This was especially a big issue on the NES, where both the game and the music is tied to the framerate of the console. The effect is obvious. Try running any NTSC (US NES or Japanese Famicom) game in an European PAL NES, and you will find the game not only running 16% slower than expected, but the music will also be slowed down, which is often more noticeable even when you haven't tried the game before.

To combat this issue, many games had their software adjusted when they were released in Europe and Australia, so that they would run slightly faster, and thus match the original NTSC timing when running on the slower PAL systems.
There is no way to brute force this solution, though. It is not possible to simply tell a game to "run faster", and a change in the game's programming is always required.
This could technically cause a lot of unexpected issues - for example the gameplay might have its speed adjusted correctly, while other less important logic such as pause menus and cutscenes still run slower. It is also not uncommon to see that only the player character has been adjusted, while everything else in the game runs at the original timing. In more egregious examples, a PAL game will actually play out completely different from its NTSC counterpart.
A popular example is Tetris, where the higher levels have the pieces move at such high speeds that it is technically impossible to adjust for the 16% difference, because the game's logic does not allow moving the piece on a "fraction of a frame".

More often however, the game's programmers would not even bother to adjust their game for the PAL release. this is especially true in older or somewhat obscure releases. But since it is usually much easier to speed up all the music in the game, than actually changing the logic of the gameplay, a lot of PAL NES games will have the music adjusted correctly, while the rest of the game still plays slower than it should.
The result is a release that will still play slower on a PAL console, but has super fast annoying music on an NTSC console.