You are right, an environment anomaly let me to falsely believe that
this had somehow affected encoding performance.

I had repeated the test many times with and without the patch using
simplejson trunk and wrongly concluded that the patch was to blame.

After correcting the environment, write performance returned to normal.

This patch seems to perform roughly the same for my decode cases, but
uses about 10-20MB less memory.  My needs are far less than that of the
other poster.

However, this bug is about the serializer (encoder).  So perhaps the
decode performance patch should be a separate bug?