I can find this in the manual:
"When Active D-Lighting is on, the capacity of the memory buffer drops
and additional time is required to record images (pg. 406). Matrix
metering is recommended (pg. 100). Although exposure is in fact
reduced to prevent loss of detail in highlights and shadows, highlights,
underexposed areas, and mid-tones are automatically adjusted to prevent
the resulting photograph from being underexposed. The [Brightness] and
[Contrast] Picture Control settings (pg. 151) can not be adjusted while active D-Lighting is in effect."
...which means the buffer fills (slightly) quicker than without Active D-Lighting which in turn reduces the burst rate, but I can't find anything about a reduced frame rate? I also looked up my BYTHOM D300 reference, nothing I can find there either.
I shoot with a grip on the D300, with Active D-Lighting on and (was) pretty sure I am getting 8fps. Although I to be honest I have never tested the actual frame rate, maybe I need to shoot a clock face over the weekend
Anyway, I reckon active D-Lighting is a beautiful thing, even if it levies a tax on the frame rate.