See for example https://perfht.ml/2vXIR2K
Since bug 1388388 we should attach a stub to call SetObjectElement directly instead of going through IonSetPropertyIC::update. It's not clear from the profile why that's not working here.
It looks like this is an object with many properties so we convert it to dictionary mode. The IC does not handle new dictionary properties so we disable it. In tryAttachAddSlotStub, we could add an IC stub that guards the object is inDictionaryMode and then we can just call SetObjectElement... Also the profile reveals a bunch more silly perf bugs on the SetProperty path that I wasn't aware of yet. Will fix.
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