That fastest change, using a Prop-1 would be about 29 milliseconds.
How? Here's a command that will affect all RC-4 pins:
SEROUT Sio, OT2400, ("!RC4", %00, "S", %1111)
If you count up the bytes in the message there are seven (four in the header, one for the address, one for the command, one for the data). At 2400 baud -- the fastest speed that the Prop-1 can use -- a byte takes about 4.17 ms to transmit; multiply this by seven bytes and you get about 29 milliseconds.
There is yet another issue: The Cyrdom relays only switch on at the zero-cross point (this reduces electrical noise and is a good thing). Since a 60 Hz signal sees the zero cross every 8.3 milliseconds the actual turn on of an output could be delayed by about that much.
Keep all this in mind if you're want to switch the RC-4 outputs quickly; the message transmission time may be enough of a delay for some applications.
And for the Prop-2/Prop-SX at 38.4K? About 1.8 milliseconds to send the message, but there could still be about an 8 millisecond delay before the actual turn-on.