Concern with Facebook's login decoding sign_request performance

I am completely new to the Facebook API. I would like to incorporate Facebook login into my application. I am using the Javascript SDK on the front-end to log the user in and retrieve the user_id and signed_request from Facebook. I then plan to send these two pieces of information via AJAX to my server (either php/hack (hhvm), node, java, or whichever language I can determine is quickest for decoding) every time my logged in user does an action on my application to validate if the user is indeed logged in and is the person they say they are. For me to accomplish this, I need to get decode the signed_request, for example in php:

function parse_signed_request($signed_request) {
  list($encoded_sig, $payload) = explode('.', $signed_request, 2); 

  $secret = "appsecret"; // Use your app secret here

  // decode the data
  $sig = base64_url_decode($encoded_sig);
  $data = json_decode(base64_url_decode($payload), true);

  // confirm the signature
  $expected_sig = hash_hmac('sha256', $payload, $secret, $raw = true);
  if ($sig !== $expected_sig) {
    error_log('Bad Signed JSON signature!');
    return null;
  }

  return $data;
}

function base64_url_decode($input) {
  return base64_decode(strtr($input, '-_', '+/'));
}

which then I will be able to extract the following JSON object:

{
   "oauth_token": "{user-access-token}",
   "algorithm": "HMAC-SHA256",
   "expires": 1291840400,
   "issued_at": 1291836800,
   "user_id": "218471"
}

to be able to compare if the user_id the user sent over matches the one in the JSON object. Then if it matches I can complete my business logic (DB manipulation).

My big concern here is a user will be sending many requests to my server, so every time I will need to decode this signed_request which can really kill my server performance. I was thinking I maybe could call Facebook from my server, pass the user_id, and receive the signed_request string, which I can then match with the signed_request string the user sent over from the client_side and see if they match. This would be more efficient, but it does not seem Facebook offers anything like this. Is there any other methods besides the heavy performing decoding to validate a user? I have gone through quite a bit of the Facebook SDK's information but could not find a solution. If I must decode, which language/library would be the best performing at this type of operation?

PS. I plan on using cordova later to create a mobile app so I must use only Javascript on the front end and can't use a server language such as php to create html for the client.