The delay? well ...
1) Android has to transmit the 1 character by bluetooth
2) the Arduino receives the character by it's wireless module. I initialize at 9600 baud, i can do it faster, but it's the advice speed..
3) The arduino has to send an order to the relay to open a 2nd circuit (DC to AC)

So, the bluetooth initialization in Arduino code is the major reason (guess is the only true reason).

And yes you can do all sort of thing with it. Just need a bit imagination, and a lot of time LOL
this one was not so hard, and i just use some basic components, the other project, the LCD with RTC clock was much more complicated.