From an initial profile in a clean local release build, it looks like the insert() method of the indirect stubs table is the problem, and more generally, that initElems takes 100% of the CPU for long stretches. For a large thread pool it could take a long time to initialize everything. Not conclusive, but very suggestive. See https://share.firefox.dev/3Imxe6a.
Bug 1750930 Comment 4 Edit History
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From an initial profile in a clean local release build, it looks like the insert() method of the indirect stubs table is the problem, and more generally, that initElems takes 100% of the CPU for long stretches, due to insert and to a very, very heavily contended lock (or a very buggy lock). For a large thread pool it could take a long time to initialize everything. Not conclusive, but very suggestive. See https://share.firefox.dev/3Imxe6a.