Fastest G protein coupled reactions
Mammalian rod cells, the most numerous photoreceptor cells in the retina, reach a max depolarization after a single photon stimulus at 100-120 ms, implying a time constant (1/k) of about 25 ms (see Chen et al., Nature 404:557).  This is on the same order of speed as the fly eye, maybe a tad faster.  Non-photoreceptor GPCRs are probably slower, but few folks have tried to measure these effects. The m1 muscarinic acetylcholine receptor catalyzes Gq activation with a time constant of about 500 ms (Mukhopadhyay and Ross PNAS 96:9539), but there may be faster ones.