Interesting test Jim. So this means that the electronic shutter in the Sony is a rolling shutter and that the second curtain is relatively slow, presumably due to read-out speed limitations ?
Exactly right. In EFCS the electronic first curtain is a rolling reset that travels across the sensor at the same speed as the mechanical first curtain. The second curtain is mechanical in both cases.
In an all-electronic shutter, the rolling reset happens as in EFCS, but slower. Instead of the mechanical second curtain, the data is read out in a rolling fashion at the same speed as the reset. The faster the readout speed, the faster the rolling reset can be. You can see from the timing that in the a7s case, it appears to be a byproduct of the camera's video capabilities.
This limitation would not apply if it were a global shutter (as longed for by Simon above). However my understanding is that a global shutter requires extra circuit elements on each sensel to allow storage of sensel voltage until read-out after the end of the shutter activation period.
That is my understanding as well.
Jim