We discussed this in #fission:matrix.org and the metrics meeting last week. AIUI, we want this probe because it can give us insight into if users will be running a large amount of documents with the same origin, which could lead to slow downs for those origins since they will all be running in a single process in the fission case (versus being spread over multiple processes in the non-fission case).
Bug 1634507 Comment 2 Edit History
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We discussed this in #fission:mozilla.org and the metrics meeting last week. AIUI, we want this probe because it can give us insight into if users will be running a large amount of documents with the same origin, which could lead to slow downs for those origins since they will all be running in a single process in the fission case (versus being spread over multiple processes in the non-fission case).