The C64's CPU wasn't slowed down much at all by the badlines, less than a few percent. I'm not sure how much the sprites slowed it. The Atari was faster but how much it was slowed by depended on the video mode. Higher res and higher colors slowed it more, leaving the CPU at around 1.3 MHz. I also don't know how much the Atari player/missile graphics slowed the CPU.