I timed the SR times for each game based on the time between when you first press the SR button combination to when you first gain control of your character ingame, assuming you're mashing A the whole time. I tested each version 3-5 times and took the average of each SR for the final times listed below. For these tests, I treated version counterparts and the same game, but third versions / sequels / remakes as separate (although all third versions ended up with the same SR time anyway). I don't have every game, so for some, I had to look up videos of shiny hunts in those games, so these times might not be as accurate. The games I was not able to personally test on were GS, RS, DP, B2W2, XY, and SM. Of these, XY were the only versions that didn't have a third version with an equal SR time that I was able to personally test on.
I didn't do Gen 1 because shiny hunting is a much more mathematically involved process, so if you're going for quick SRs, you probably wouldn't hunt in Gen 1 (plus I don't have any Gen 1 games).
GS/C (A+B+start+select): 8.5 seconds
GS/C (VC menu): 11.2 seconds
RS/E (with working battery): 6.6 seconds
RS/E (with dry battery): 12 seconds
FRLG: 10 seconds
DP/Pt: 16.7 seconds
HGSS: 16.9 seconds
BW/B2W2: 20.9 seconds
XY: 14.2 seconds
ORAS: 20.7 seconds
SM/USUM: 12.2 seconds
LGPE: 38.7 seconds
SwSh: 27.5 seconds
The shortest SR time is RSE, without the dry battery prompt, at 6.6 seconds, while the longest is LGPE.