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Assuming a simple parallel scheme as in your question, in my opinion you should not observe such a framerate drop.

I see multiple flaws in your code:

usage of deprecated C API as pointed by berak
a mutex is missing even if in this simple code it should not matter
you should not time the grabbing but rather the whole acquisition as the camera acquisition should be done in background and each new frame should be stored in a buffer (if I am not wrong)

With the following code, I observe the same framerate with or whitout a parallel thread launched:

#include <iostream>
#include <thread>
#include <mutex>
#include <opencv2/opencv.hpp>

using namespace std;
using namespace cv;

mutex quit_mutex;
bool quit = false;

void thread_callback() {
  while (true) {
    lock_guard<mutex> lock(quit_mutex);
    if (quit)
      break;
  }
}


int main(int argc, char *argv[]) {
  VideoCapture capture(0);
  capture.set(CAP_PROP_FRAME_WIDTH, 1280);
  capture.set(CAP_PROP_FRAME_HEIGHT, 720);

  if (!capture.isOpened()) {
    cerr << "Cannot open camera!" << endl;
    return 0;
  }

  thread th;
  bool launch_thread = argc > 1 && atoi(argv[1]);
  if (launch_thread) {
    th = thread(thread_callback);
  }

  Mat frame;
  double t = (double) getTickCount();
  int iterations = 0;
  while (true) {
    capture >> frame;
    imshow("Camera", frame);

    int c = waitKey(30) & 0xFF; // & 0xFF should be useless with OpenCV >= 3.2
    iterations++;
    if (c == 27)
      break;
  }
  t = ((double) getTickCount() - t) / getTickFrequency();
  cout << "Mean capture time: " << iterations / t << " FPS on " << iterations << " iterations" << endl;

  if (launch_thread) {
    //Set quit to true
    unique_lock<mutex> lock(quit_mutex);
    quit = true;
    lock.unlock();
    th.join();
  }

  return 0;
}

To enable the thread launching, you have to pass as argument 1 to the executable.

Some additional links related to the subject:

Assuming a simple parallel scheme as in your question, in my opinion you should not observe such a framerate ~~drop.~~drop on a modern CPU.

I see multiple flaws in your code:

usage of deprecated C API as pointed by berak
a mutex is missing even if in this simple code it should not matter
you should not time the grabbing but rather the whole acquisition as the camera acquisition should be done in background and each new frame should be stored in a buffer (if I am not wrong)

With the following code, I observe the same framerate with or whitout a parallel thread launched:

#include <iostream>
#include <thread>
#include <mutex>
#include <opencv2/opencv.hpp>

using namespace std;
using namespace cv;

mutex quit_mutex;
bool quit = false;

void thread_callback() {
  while (true) {
    lock_guard<mutex> lock(quit_mutex);
    if (quit)
      break;
  }
}


int main(int argc, char *argv[]) {
  VideoCapture capture(0);
  capture.set(CAP_PROP_FRAME_WIDTH, 1280);
  capture.set(CAP_PROP_FRAME_HEIGHT, 720);

  if (!capture.isOpened()) {
    cerr << "Cannot open camera!" << endl;
    return 0;
  }

  thread th;
  bool launch_thread = argc > 1 && atoi(argv[1]);
  if (launch_thread) {
    th = thread(thread_callback);
  }

  Mat frame;
  double t = (double) getTickCount();
  int iterations = 0;
  while (true) {
    capture >> frame;
    imshow("Camera", frame);

    int c = waitKey(30) & 0xFF; // & 0xFF should be useless with OpenCV >= 3.2
    iterations++;
    if (c == 27)
      break;
  }
  t = ((double) getTickCount() - t) / getTickFrequency();
  cout << "Mean capture time: " << iterations / t << " FPS on " << iterations << " iterations" << endl;

  if (launch_thread) {
    //Set quit to true
    unique_lock<mutex> lock(quit_mutex);
    quit = true;
    lock.unlock();
    th.join();
  }

  return 0;
}

To enable the thread launching, you have to pass as argument 1 to the executable.

Some additional links related to the subject:

Time delay in VideoCapture opencv due to capture buffer