Using test equipment, I'd imagine the way to do it would be to sync a microphone and a logic analyzer and then short the shutter button. For the Sony A7 Mark I it's around 0.023 seconds : https://www.imaging-resource.com/PRODS/sony-a7/sony-a7A6.HTM

Granted, it's not "no shutter lag", but 2.3ms might as well not exist.