performance drop with webpack 4 in production mode

[IG-88-2]

Description: when you build react-vis with latest webpack in production mode each call to handleNearestXY will take 4-5 times longer on average. I have been able to narrow it down to one of the dependencies. Voronoi instance generation time on each mouse move changes dramatically with mode production.
Problem occur here (d3-voronoi Diagram constructor):

  while (true) {
    circle = firstCircle;
    if (site && (!circle || site[1] < circle.y || (site[1] === circle.y && site[0] < circle.x))) {
      if (site[0] !== x || site[1] !== y) {
        addBeach(site); //This guy
        x = site[0], y = site[1];
      }
      site = sites.pop();
    } else if (circle) {
      removeBeach(circle.arc); //And his friend
    } else {
      break;
    }
  }

After trying different sets of options i have been able to remove unwanted behavior setting unused option in uglifyOptions to false:

 plugins:[
        argv.env==="development" ? null : 
        new UglifyJsPlugin({
            uglifyOptions:{
                compress:{
                    unused:false
                }
            }...

which magically changes performance back to normal. Although this fixes my problem, I'm still struggling to understand what is going on and why this problem arise. I created repo to demonstrate issue:
https://github.com/WildHorse19/react-vis-uglify-performance-drop
In order to see difference just do npm run slow, npm run fast and simply try to drag mouse pointer.
The reason i decide to mention this here is because its really hard to spot this one, webpack comes with set of defaults in production mode and so happens that compression and unused:true is one of the defaults, also this wont be noticeable with small data sets.