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: 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. This only applies to non-repeating background layers. Repeating backgrounds and sprites don’t require extra CPU power to make them move fast.