karma.config-part.js

    ...
    frameworks: [
        'parallel',
        'jasmine'
    ],
    plugins: [
        "karma-jasmine",
        "karma-chrome-launcher",
        "karma-junit-reporter",
        "karma-spec-reporter",
        "karma-parallel"
    ],
    parallelOptions: {
        executors: (Math.ceil(require('os').cpus().length / 2)),
        shardStrategy: 'round-robin'
    },
    ...