FPS drop in low light even with manual focus?

[Hjones]

Do you have in-camera noise reduction turned on? Perhaps the metering is slower in the dark even though the camera is in manual mode the meter is still working, I am not sure of the reason, it does seem odd. Do you have an external flash attached?

I have noise reduction off and I don't have an external flash attached-- I feel like the most plausible explanation would be the meter like you said, but part of me feels like there couldn't be such a huge drop in FPS just from the meter trying to function. Though, like I said, this is an extreme case, since it's comparing a near-black room to a bright monitor screen.