It's weird that the debugger works slow (almost not work!!!)

Type: Debugger

Describe the bug

• OS and Version: MacOS 11.1
• VS Code Version: 1.54.3
• C/C++ Extension Version: 1.0.0
• Other extensions you installed (and if the issue persists after disabling them):
  CodeLLDB 1.6.1 , Code Runner 0.11.3
• A clear and concise description of what the bug is.
  When I finished the c_cpp_properties.json, tasks.json and launch.json according to the offical site "Using Clang in VS Code" with my own cpp.
  The compile and output progress is right. However when I use debugger and enter the main(), when I press the "step debug" button, the speed of the debugger is really slow.
  example: the debugger debug an "string my_str" use almost 3mins!!!!

code

#include <iostream>
#include <unordered_map>
#include <string>
#include <random>
#include <ctime>
#define NUM 200000
using namespace std;

string strRand(int length); 

int main(){
    unordered_map<string, int> my_map;
    string my_str[NUM];
    for(auto it: my_str)
        it = strRand(10);
    for(int i=0;i<10;i++)
        cout << my_str[i] << endl;
    // cout << strRand(10) << endl << strRand(10) << endl << strRand(10) << endl;
    return 0;
}

string strRand(int length){
    char tmp;
    string buffer;

    random_device rd;  
    default_random_engine random(rd());  

    for(int i=0;i<length;i++){
        tmp = random() % 36; 
        if(tmp<10) 
            tmp+= '0';
        else{ 
            tmp -= 10;
            tmp += 'a';
        }
        buffer += tmp;
    }
    return buffer;
}

launch.json

{
    // Use IntelliSense to learn about possible attributes.
    // Hover to view descriptions of existing attributes.
    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
    "version": "0.2.0",
    "configurations": [
      {
        "name": "clang++ - Build and debug active file",
        "type": "cppdbg",
        "request": "launch",
        "program": "${fileDirname}/${fileBasenameNoExtension}",
        "args": [],
        "stopAtEntry": true,
        "cwd": "${workspaceFolder}",
        "environment": [],
        "externalConsole": false,
        "MIMode": "lldb",
        "preLaunchTask": "clang++ build active file"
      }
    ]
  }

Start by reproducing the slowdown with the provided C++ program and launch.json on macOS 11.1 using VS Code 1.54.3 and the C/C++ extension 1.0.0. Compare stepping into main() and the string my_str declaration, while checking the listed CodeLLDB and Code Runner extensions. Done means identifying a repeatable debugger cause and confirming improved stepping behavior.