I used two Icon On/Off modules on a test stand. I don't use dimmers and the one I have has the famous bad tact switch syndrome.
Isolated from the power lines with an XPPF.
Factory reset between tests.
Cross linked so each can control the other through Insteon. There is a very slight pause between the controlling switch and the responding switch. My installed SwitchLinc Relay linked to an ApplianceLinc shows this slight delay also. Probably an Insteon thing. Like fast on and off is a quick tap on the paddle.
X10 addresses only in each one. Definite maybe one second delay between the controlling switch and the responding switch.
X10 address in each one and cross linked Insteon. Slight delay as seen in the Insteon cross linked only.
At one time I think I had an Insteon link in one direction and X10 in both. That resulted in a slow response in the direction not having a Insteon link. Not 100% on this last test.