See https://github.com/whatwg/fetch/pull/1556
Note: WebKit has chosen to make them not exposable across origins even with TAO and CORS. See https://github.com/w3c/server-timing/issues/89
More specifically https://github.com/w3c/resource-timing/issues/342
Right, it was a TPAC subject, I forgot. Guess this isn’t relevant for WebKit.