The speed is primarily limited by the rate at
which the signals can be transmitted over the
power line. Each component of the
transmission takes about 0.6 seconds, so:
Address A1, A On, Address A1, A Off
would take about 2.4 seconds.
But if you wanted to make just a "flasher",
you could eliminate all but the very first
address and afterwards send just
A On, A Off, A On, A Off, ...
and cut the cycle time to about 1.2 seconds.