No, it happens instantly. Adding throttling would be easy enough, I just didn't bother putting it in since I don't know what the actual speeds are and they'd vary considerably between different manufacturers.
Don't quote me on this but I'm guessing the real-time speed wouldn't be too different anyway, the majority of time is spent streaming 512-byte blocks across the SPI bus which is already being throttle back for the sake of the slow LCD displays. Anyone doing time-critical SD streaming might want to take this into account, I don't see why you couldn't set SPI back to full-speed for SD transfers.
Rodot, have you done any tests with the SD cards running at full speed?