Isn't the obvious solution here to just cache the delay? The browser presumably knows how long the original resource request took when site A made it, so it can just expose the same latency to site B and - since the cache by definition only hits if the request is identical - site B can't tell whether it's delayed by artificial cache latency or actual network latency. Chuck a symlink to the resource into each per-origin cache so non-initial requests still finish immediately, and possibly add a small chance of silently retrying the underlying request to see if it gets faster (with probability that can be computed entirely from site-B-observable load times).