Mongoose for http server looks the easiest option by far. Two files!
Has anyone here used Mongoose? because looking at it briefly it seems to work as web server but I am not seeing anyway to make requests using Mongoose.
Looks like you may plug PHP into Mongoose and then you can do whatever you like with it, have local DB in a file with SQLite or execute other remote requests, or even call localhost process written in another language. On the plus side PHP may be easier to understand to other developers who will maintain this code than a custom web server written with C++.
I am going to try to give some approximate idea of the performance versus time. If performance is super-critical - you need your fastest response to be quicker than 10 ms (without a DB), then you would write your web server with some socket library in C++, like mentioned boost::asio. But you are looking at more than 10 times longer development time compared to running a PHP installation, and a maintenance hell later. That is my humble estimation. Multiply it by Pi.
Of course I am pulling these numbers out of whatever. It depends on your task.