Error message takes a long time to load

public class SignUpActivity extends ActionBarActivity {

    protected EditText mUsername;
    protected EditText mEmail;
    protected EditText mPassword;
    protected Button mSignupButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sign_up);

        mUsername = (EditText)findViewById(R.id.usernameField);
        mEmail = (EditText)findViewById(R.id.emailField);
        mPassword = (EditText)findViewById(R.id.passwordField);
        mSignupButton = (Button)findViewById(R.id.signupButton);

        mSignupButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String username = mUsername.getText().toString();
                String email = mEmail.getText().toString();
                String password = mPassword.getText().toString();

                username = username.trim();
                password = password.trim();
                email = email.trim();

                if (username.isEmpty() || email.isEmpty() || password.isEmpty()) {
                    AlertDialog.Builder builder = new AlertDialog.Builder(SignUpActivity.this);
                    builder.setMessage("Please make sure you enter a username, email, and password.")
                           .setTitle("Error!")
                           .setPositiveButton(android.R.string.ok, null);
                    AlertDialog dialog = builder.create();
                    dialog.show();
                } else {
                    //create new user!
                    ParseUser newUser = new ParseUser();
                    newUser.setUsername(username);
                    newUser.setEmail(email);
                    newUser.setPassword(password);
                    newUser.signUpInBackground(new SignUpCallback() {
                        @Override
                        public void done(ParseException e) {
                            if (e == null) {
                                Intent intent = new Intent(SignUpActivity.this, MainActivity.class);
                                intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                                intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
                                startActivity(intent);
                            } else {
                                AlertDialog.Builder builder = new AlertDialog.Builder(SignUpActivity.this);
                                builder.setMessage(e.getMessage())
                                        .setTitle("Error!")
                                        .setPositiveButton(android.R.string.ok, null);
                                AlertDialog dialog = builder.create();
                                dialog.show();
                            }
                        }
                    });
                }
            }
        });
    }