Am working through Hummingbird and am running into a number of issues. I don't expect you to help me solve them right now, but wanted to pass on that Stripe.js and Hummingbird are not getting along very well.
Obviously, for those using Stripe as a payment gateway for Pro-Sites or Membership, this can be problematic.
I'm still committed to the process, but it's difficult to troubleshoot images like that on test site, because the payment gateway is only going to fire on a live one.
In terms of performance, when I don't have hummingbird actively minifying, my Google Pagespeed report has 29 blocking scripts and 36 blocking CSS resources, for a report of 62/100. With Hummingbird, those numbers go way down (about 11 each) for a score of 65/100 (largest bogies are buddypress avatar image compression and gravatars). I'm hoping that with some suggested changes to SmushPro the BP images, at least, can be addressed.