These are myths that frequently show up that mislead people into writing bad code based on false perception of what the system actually does.
Myth: The PPU steals cycles from the CPU if there are a lot of sprites on-screen.
Fact: The PPU does not steal cycles. What takes CPU time is calculating where each sprite is moving and writing the sprites to the OAM. If you need to use a lot of sprites for something, you might as well go for it.
Myth: The SNES has only 32KB of audio RAM.
Fact: The SNES has 64KB of audio RAM.
Myth: Scrolling slower prevents slowdown.
Fact: It's not quite that simple, as scrolling speed doesn't affect CPU speed. What takes CPU time is preparing updates at the leading edge of the viewport every time the camera reaches a new row or column of the nametable. This involves decoding map data from ROM into a buffer in RAM, and scrolling faster means this happens more often.
Actually, this only consumes a lot of CPU time when a level map is being heavily decompressed in real time. Otherwise just go with a maximum scroll speed of 8 or 16 pixels per frame and you'll be fine. Using 16x16 tile mode does help a lot with this.
Myth: You need to NOP after firing off a DMA
Fact: Licensed games like Actraiser and Breath of Fire do not NOP after DMA.
Myth: Screen Pixelation is Mode 7
Fact: The Screen Pixelation effect seen when selecting a level in Super Mario World is done by manipulating the Mosaic register at 2106h. The 3D flying effect seen in Squaresoft RPGs is controlled by the BGMODE register at 2105h and this uses Mode 7 to dusplay the ground. Many racing games, such as Super Mario Kart and F-Zero, use Mode 7, and texels closer to the player appear bigger than ones near the horizon due to HDMA doing the job of the perspective divide, but this is not the mosaic effect.