massive speedup on temp table creation/destruction?

I was just trying (unsuccessfully so far) to replicate Csaba Nagy's
report of a strange failure with temp table creation.  I made use
of pgbench's recent improvements to be able to push random scripts
at a collection of backends:

$ cat ttscript.sql
create temp table foo (f1 int);
drop table foo;
$ pgbench -n -c 98 -t 1000 -f ttscript.sql bench

This is just pushing a long string of create temp table/drop table
commands at a whole lot of backends concurrently.

What I found surprising is that I get numbers like 430 tps from HEAD
and 220 tps from REL8_1_STABLE.  This test case is of course not about
performance, but I'm not quite sure what we changed that would produce
a 2X speedup from 8.1.  Can anyone else replicate this?
        regards, tom lane