custom less plugin. instead of compiling each less file and concatenating, first concatenate and then compile. much faster.

less-plugin.js

'use stric';

var fs = require('fs');
var less = require('less');
var path = require('path');
var async = require('async');
var _ = require('lodash');

function readLessFilesIntoString(lessFiles, callback) {
    var initialText = '';
    async.reduce(lessFiles, initialText, function(memo, item, cb){
        fs.readFile(item, 'utf8', function (error, data) {
            if (error) throw error;
            cb(null, memo + data);
        });
    }, function(err, lessFilesContents){
        callback(null, lessFilesContents);
    });
}

function runLess(lessInput, options, callback) {
    less.render(lessInput, options).then(function (output) {
        callback(null, output.css);
    }).catch(function (error) {
        console.log(error);
    });
}

function addToAssets(compilation, css, outputFile, callback) {
    compilation.assets[outputFile] = {
        source: function() {
            return css;
        },
        size: function() {
            return css.length;
        }
    };
    callback();
}

function LessPlugin(lessOptions, outputFile) {
    this.lessOptions = lessOptions;
    this.outputFile = outputFile;
}

LessPlugin.prototype.apply = function(compiler) {
    var lessOptions = this.lessOptions;
    var outputFile = this.outputFile;
    compiler.plugin('emit', function(compilation, callback) {
        var lessFiles = [];

        var appChunk = compilation.chunks.filter(function (chunk) {
            return chunk.name === 'app';
        }).pop();

        appChunk.modules.forEach(function(module) {
            module.fileDependencies = module.fileDependencies || [];
            module.fileDependencies.forEach(function(filepath) {
                if (/\.less$/.test(filepath)) {
                    lessFiles.push(filepath);
                }
            });
        });

        async.waterfall([
            function (cb) {
                readLessFilesIntoString(lessFiles, cb);
            },
            function (lessContent, cb) {
                runLess(lessContent, lessOptions, cb);
            },
            function (cssContent, cb) {
                addToAssets(compilation, cssContent, outputFile, cb);
            }
        ], function () {
            callback();
        });

    });

};

module.exports = LessPlugin;