IO speed doesn't take shared hit into account

[pgiraud]

Describe the bug

In the following plan, when computing IO read speed, the hit is not taken into account. It shows 38GB, but it should be 39GB. Speed should be 273 MB/s instead of 270 Mb/s.

https://explain.dalibo.com/plan/69781d46bgc6e2ea#plan/node/2

For reference, see https://www.depesz.com/2026/02/21/per-worker-and-global-io-bandwidth-in-explain-plans/