I have no idea why publish WebCam in delayed image

edit

i'm work in ubuntu 20.04, ROS2 Foxy, C++ enviroment. what i'm trying to do is try to publish webcam and subscride it. problem is i succesfully publish webcam but not in realtime. image is publish every 2 seconds. i have no idea why this happen i'm post my publish, subscribe code right here.

this is publish node

#include <chrono>
#include <memory>
#include "cv_bridge/cv_bridge.h"
#include "rclcpp/rclcpp.hpp"
#include "sensor_msgs/msg/image.hpp"
#include "std_msgs/msg/header.hpp"
#include <opencv2/opencv.hpp>
#include <stdio.h>
#include <opencv2/core/types.hpp>
#include <opencv2/core/hal/interface.h>
#include <image_transport/image_transport.hpp>
#include <opencv2/imgproc/imgproc.hpp>

using namespace std::chrono_literals;
using namespace cv;

class MinimalPublisher : public rclcpp::Node {

public:

MinimalPublisher() : Node("minimal_publisher"), count_(0) {

    auto qos_profile = rclcpp::QoS(rclcpp::KeepLast(10));
    publisher_ = this->create_publisher<sensor_msgs::msg::Image>("topic", qos_profile);
    timer_ = this->create_wall_timer(10ms, std::bind(&MinimalPublisher::timer_callback, this));

}

private:

void timer_callback() {

    cv_bridge::CvImagePtr cv_ptr;
    cv::VideoCapture cap(0);
    cv::Mat img(cv::Size(1280, 720), CV_8UC3);
    cap >> img;
    sensor_msgs::msg::Image::SharedPtr msg = cv_bridge::CvImage(std_msgs::msg::Header(), "bgr8", img).toImageMsg();      
    publisher_->publish(*msg);
    RCLCPP_INFO(this->get_logger(), "publishing");
}
  rclcpp::TimerBase::SharedPtr timer_;
  rclcpp::Publisher<sensor_msgs::msg::Image>::SharedPtr publisher_;
  size_t count_; 
};

int main(int argc, char *argv[]) {

  printf("Starting...");
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<MinimalPublisher>());
  rclcpp::shutdown();
  return 0; 
}

this is subscriber node

#include <chrono>
#include <memory>

#include "cv_bridge/cv_bridge.h"
#include "rclcpp/rclcpp.hpp"
#include "sensor_msgs/msg/image.hpp"
#include "std_msgs/msg/header.hpp"
#include <opencv2/opencv.hpp>
#include <opencv2/highgui.hpp>
#include <stdio.h>

#include <opencv2/core/types.hpp>
#include <opencv2/core/hal/interface.h>
#include <image_transport/image_transport.hpp>
#include <opencv2/imgproc/imgproc.hpp>

using std::placeholders::_1;

class MinimalSubscriber : public rclcpp::Node {
  public:
    MinimalSubscriber()
    : Node("minimal_subscriber")
    {
      auto qos_profile = rclcpp::QoS(rclcpp::KeepLast(10));
      subscription_ = this->create_subscription<sensor_msgs::msg::Image>(
      "topic", qos_profile, std::bind(&MinimalSubscriber::topic_callback, this, _1));
    }

  private :
    void topic_callback(sensor_msgs::msg::Image::SharedPtr msg) const
    {
      RCLCPP_INFO(this->get_logger(), "In callback");
      cv_bridge::CvImagePtr cv_ptr;
      cv_ptr = cv_bridge::toCvCopy(msg,"bgr8");
      cv::imshow("minimal_subscriber", cv_ptr->image);
      cv::waitKey(1);
    }
    rclcpp::Subscription<sensor_msgs::msg::Image>::SharedPtr subscription_;
};

int main(int argc, char *argv[])
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<MinimalSubscriber>());
  rclcpp::shutdown();
  return 0;
}