Time from stimulus to gene expression
The fastest I know of is the heat shock locus in Drosophila.  The transcription factor (HSF) accumulates within about 30 seconds, and RNA Polymerase can be seen to start accumulating within 3 minutes.

Katie L. Zobeck, Martin S. Buckley, Warren R. Zipfel, John T. Lis.  2010. Recruitment Timing and Dynamics of Transcription Factors at the Hsp70 Loci in Living Cells.  Molecular Cell 40(6): 965-975