It simply does not matter if it takes 0.1 ms, 1 ms, 10 ms or 100 ms to switch the relais

Too right...the script to open/close the curtains will probably run from CRON every 10 minutes or so with an updater script running from CRON a couple of times a day. So timing is not critical.

Just make sure the relais can switch the current (A) and voltage (V) required by the electric curtains

The curtain motor is 12V 1A so no worries there. It probably doesn't actually need a relay as a thyristor should do it but relays are simpler as I won't have much access to the motor setup whilst building the controller.