What am I doing wrong?

That's a strange one - have you tried maybe updating the screen on every second GPS update? If the update speed is too high, that may help.

Otherwise, I've got a new version of the firmware available here which I plan to release soon. The Serial API is slightly re-done (sending strings, not characters), so the GPS receiver part now looks like:

exports.connect = function(serial, callback) {
  var gps = {line:""};
  serial.on('data', function(data) {
    gps.line += data;
    var idx = gps.line.indexOf("\n");
    while (idx>=0) {
      var line = gps.line.substr(0, idx);
      gps.line = gps.line.substr(idx+1);
      handleGPSLine(line, callback); 
      idx = gps.line.indexOf("\n");     
    }
  });
  return gps;
}

That should handle significantly higher data rates. I don't see 57600 being a problem at all.