Slow Autocomplete for translation units including bulky header files

Type: LanguageService

Describe the bug

• OS and Version: macOS Mojave (10.14.5), MBP Mid 2015
• VS Code Version: 1.36.0
• C/C++ Extension Version: 0.24.0
• Other extensions you installed (and if the issue persists after disabling them): CMake Tools, Git lens
• Autocompletion for source files including bulky header files like the ones given below. Auto complete sometimes takes 5 seconds to return result. Most of the time it steadily takes 2-3 seconds to return results.
  Tried disabling/enabling IntelliSense cache yet 4-5 seconds lag was visible.

To Reproduce
Source Sample:

#include <algorithm>
#include <atomic>
#include <chrono>
#include <cstdio>
#include <cstdlib>
#include <iomanip>
#include <iostream>
#include <random>
#include <thread>
#include <vector>

#include <boost/intrusive/list.hpp>
#include <boost/lockfree/queue.hpp>
#include <boost/program_options.hpp>

Preprocessed file has 1.3 lakh lines (4.4 MB).
c_cpp_properties.json:

{
    "configurations": [
        {
            "name": "Mac",
            "includePath": [
                "${workspaceFolder}/**",
                "${vcpkgRoot}/x64-osx/include"
            ],
            "defines": [],
            "macFrameworkPath": [
                "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/System/Library/Frameworks"
            ],
            "compilerPath": "/usr/bin/clang",
            "cStandard": "c11",
            "cppStandard": "c++17",
            "intelliSenseMode": "clang-x64",
            "configurationProvider": "vector-of-bool.cmake-tools",
            "compileCommands": "${workspaceFolder}/build/compile_commands.json"
        }
    ],
    "version": 4
}

Expected behavior
Autocompletion returning results within a second always (too much to ask for?).

Screenshots
Instead autocomplete taking 5 seconds to complete

Profiler Output
Just in case, if you may require, I've attached Instruments profile output of server for autocomplete operation.
Run 1 -> IntelliSense caching disabled.

Run 2 -> IntelliSense caching enabled.

Complete Browsable Instruments Trace:
Msvc.Autocomplete.trace.zip

Start with the C++ source sample, c_cpp_properties.json, and the attached Msvc.Autocomplete.trace.zip profiler output. Reproduce autocomplete on the listed standard and Boost headers, then compare the observed 2–5 second latency with the expected response within one second.