This was spun off  issue 690428 , specifically https://bugs.chromium.org/p/chromium/issues/detail?id=690428#c11. That issue was supposed to cover updating our JS-sequence conversion code and make it work with the latest WebIDL spec, in that we now support custom iterators and no longer have a special case for deprecated WebIDL arrays.

That has been done, but simply converting all JS objects (including arrays) to sequences via https://heycam.github.io/webidl/#create-sequence-from-iterable is too slow (see https://bugs.webkit.org/show_bug.cgi?id=166834#c13, for example), so at least Gecko and WebKit implement some fast-path optimizations for e.g. arrays with no custom iterators or weird properties.

We're lagging behind here, as the work in  issue 690428  was focused on supporting the new sequence conversion steps while leaving the previous "optimization" path intact. This is problematic, as it amounts to basically calling v8::Value::IsArray() and iterating over it in a loop. We also use this path even if an array has a custom @@iterator symbol defined.

We need to take a lot more into consideration when deciding whether to take shortcuts in the sequence conversion steps. For example, https://github.com/w3c/web-platform-tests/issues/5566#issuecomment-294181173 says "One thing that needs to be done carefully there (apart from not optimizing cases which have non-default iterator behavior) is either not optimizing arrays with accessors or handling changes to %ArrayIterator%.prototype.next partway through the iteration, when a getter for an accessor property on the array mutates it".

Fixing this probably needs some work in V8 as well, or we risk being as slow as the code path for custom iterators we currently implement in the bindings code (it would also be great if V8 could do some of the work we currently do in NativeValueTraits<IDLSequence<T>>::ConvertSequenceSlow(), https://cs.chromium.org/chromium/src/third_party/WebKit/Source/bindings/core/v8/NativeValueTraitsImpl.h?type=cs&q=nativevaluetraitsimpl.h+package:%5Echromium$&l=343).