All the Workers stuff runs on a machine that your request was already going to pass through anyway, and does very little actual work compared to everything your

All the Workers stuff runs on a machine that your request was already going to pass through anyway, and does very little actual work compared to everything your client machine has to do to render a complex site. I would guess that part of the work is easily offset. The only question is really step 2: How much work is required to communicate with the CO2signal API? If that API is also in a Worker and/or is able to make good use of the local Cloudflare cache then it might be quite cheap. If it has to talk to an external server somewhere then it's hard to say. With all that said, presumably the net effect is negligible either way and the real goal is to raise awareness and demonstrate how these APIs could be used. (disclosure: I'm the lead engineer for Cloudflare Workers.)