ember cli build slow with babel includePolyfill

why is babel so slow with includePolyfill: true

I have latest version of "ember-cli": "2.3.0" and "ember-cli-babel": "^5.1.5"

Build successful - 126129ms.

Slowest Trees                                 | Total
----------------------------------------------+---------------------
Babel                                         | 38205ms
JSHint app- QUnit                             | 36606ms
Babel                                         | 29282ms

Slowest Trees (cumulative)                    | Total (avg)
----------------------------------------------+---------------------
Babel (14)                                    | 77556ms (5539 ms)
JSHint app- QUnit (1)                         | 36606ms

without polyfill

Build successful - 6896ms.

Slowest Trees                                 | Total
----------------------------------------------+---------------------
Babel                                         | 452ms

Slowest Trees (cumulative)                    | Total (avg)
----------------------------------------------+---------------------
Babel (14)                                    | 1797ms (128 ms)
Funnel: Addon JS (9)                          | 555ms (61 ms)