Re: What's the problem?

[Richard Stallman]

    I think people on emacs-devel agreed that private binding stacks would
    work, but their concern was that it would slow down Emacs a lot if
    every read/write did a binding lookup (I am probably remembering
    things incorrectly, but that was my understanding).

The values would be stored in the symbols, just as now.
However, thread-switching would have to swap bindings in and out.

Here's what I sent privately:

This could work using shallow binding, if switching threads swaps
out the bindings that shouldn't apply, then swaps in those that
should.  This is similar to the technique now used for buffer-local
bindings when switching buffers; however, for buffer-switching, the
swapping is done in a lazy fashion, but I don't see how that could
be possible for thread-switching.  So thread-switching would take
some time.  We would want to make sure it does not happen very
often.