Huge TTFB using CDN (CloudFront)

Hi,

We created a CloudFront distribution to serve our static assets (https://assets.koolicar.com)
We always have “x-cache: Hit from cloudfront” but TTFB (Time To First Byte) is huge (>3s per img/css)
https://www.webpagetest.org/result/180201_AD_556c56df595a618cfa232f2cc8f035ae/1/details/#waterfall_view_step1

Any idea?