Most Western systems suffered from a severe lack of memory bandwidth. Japanese systems tended to have two separate buses, one for the CPU and one for the video. It was necessary early on in order to display the more complex Japanese characters.
By having two buses, the graphics could be a lot better than on systems that shared a single bus between the CPU and graphics. The Famicom managed to have a decent number of colours and sprites on screen at a good resolution, while others had to compromise at least one of those things. It did mean that the cartridge needed two ROM chips, but it was well worth it.
It also meant that the CPU was faster. A 6502 derivative which most Western systems would only operate at a maximum of 1MHz, so that 2MHz RAM and ROM chips could alternate between servicing the CPU and graphics chip. The 6502 was very efficient anyway, so it was actually quite a powerful little system. The later Super Famicom only doubled the speed, but retained the 8 bit CPU with a few minor 16 bit extensions.