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.
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.