Remote control: receiving the command
it's a bit hard to comment on your question as it's really very general.
You say that the device should receive commands as fast as possible and with very few data and than you ask "Does it overload the server? Is it fast enough for the client?", but you don't mention what kind of operations would be actually performed and in what way. The process of receiving and parsing command is usually much faster than the process of executing parsed command. 

As for the code itself, the client side looks a bit like reinventing the wheel - why don't you try jQuery?
As for the server code (content.php), it's hard to tell what it does other than it obviously tries to read a file from disk serveral times, but even if succeeds - it still does nothing...

Maybe if you'd ask more specific questions, I could be more of a help.