I am told it involves less copies than WriteBytes/ReadBytesInto.

How/why?

Quoting Nika:

If the buffer is a sequence of trivial numeric types, and the length is over 64k bytes, it [WriteSequenceParams] allocates a temporary shared memory region and sends that instead.

WriteBytes copies the data into the IPC::Message's buffer, and then that is copied into a system pipe, the nout of it into another IPC::Message, and then finally read into your final thing. IPC::Message uses a BufferList right now, meaning that it serilizes data into 4k chunks

Are there benchmarks that show the advantages of the shared memory approach?

Talos tests don't seem to be affected by the change:

• https://treeherder.mozilla.org/perfherder/compare?originalProject=try&originalRevision=3478f4cb30eb75575e8fd338cea5029c7f09127a&newProject=try&newRevision=1b07ad0f6b70665a33f85f86f4405ee0f4b84c83&framework=1&page=1
• https://treeherder.mozilla.org/perfherder/compare?originalProject=try&originalRevision=4e6b698022fbdbc823225483715b2e9f6fb34551&newProject=try&newRevision=8bc537a5ca24898eb339531d8bdb3eb6b7fcc9e8&framework=1&page=1

Quoting Mika again:

if the display lists tend to be under 64k it probably won't change much at all

Nope, there aren't really any major benchmarks showing improvements there. It's mostly a lever to consider pulling if we think we'll see performance improvements from the change due to seeing the existing memcpys in profiles.

I think for extremely large messages (megabytes) we could see some improvements, but I wouldn't expect to see much change at all for reasonably sized messages.