In theory could be as low as single digit ms overhead, assuming fastly and cloudflares PoPs being used are very close to each other. In reality it seems higher than that but I'm sure a lot of optimisations can be done.

While this was measured in a slightly different implementation via Oblivious DNS[1]:

> The first thing that we can say with confidence is that the additional encryption is marginal. We know this because we randomly selected 10,000 domains from the Tranco million dataset and measured both encryption of the A record with a different public key, as well as its decryption. The additional cost between a proxied DoH query/response and its ODoH counterpart is consistently less than 1ms at the 99th percentile.

[1]: https://blog.cloudflare.com/oblivious-dns/#what-about-perfor...