UserAgent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/65.0.3294.1 Safari/537.36

Steps to reproduce the problem:
1. Make sure experimental canvas features is enabled (for OffscreenCanvas to be available)
2. Visit https://www.scirra.com/labs/c3worker/dom/ (this renders via HTML canvas)
3. Observe performance; take a performance profile in dev tools and look at GPU thread
4. Visit https://www.scirra.com/labs/c3worker/worker/ (this renders via OffscreenCanvas in a worker; the code is identical except for the OffscreenCanvas rendering path)
5. Observe performance; take a performance profile in dev tools and look at GPU thread

What is the expected behavior?
Rendering via an OffscreenCanvas from a worker should perform as well as rendering via a HTML canvas from the main document.

What went wrong?
Rendering via OffscreenCanvas from a worker has significantly worse performance. On a high-end laptop, the second URL is visibly choppy and the dev tools performance profile shows the GPU is pretty much 100% busy. In the HTML canvas version the framerate is perfectly smooth and shows ~1ms GPU work per frame.

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 65.0.3294.1  Channel: canary
OS Version: 10.0
Flash Version: