Following askvortsov suggestion, I implemented gzip support and reporting to the Lab.
This means the Lab will now accept gzip for all requests, which can potentially speed up the scan for websites hosted on slower connections or on the other side of the globe (all requests are made from a DigitalOcean Droplet in Germany).
The Lab will also show show whether gzip is supported in the "Assets size" box and how much bandwidth you are saving.
If the server doesn't support gzip, a message will appear in the same box with recommendations on how to enable it.