V3 protocol is slower than V2

Hi,

V3 protocol seems to be slower than V2 if we don't use prepared statement
together. I measured performance of protocols on 8.3beta using pgbench -S
with modification to use PQsendQueryParams and PQsendQueryPrepared
instead of PQsendQuery. (I'll send the patch to -patches shortly.)
 [V2]  PQsendQuery (V2; original) [V3]  PQsendQueryParams (V3) [V3P] PQsendQueryPrepared (V3 with prepared statements)

V3 was 12% slower than V2, and V3P was 40% faster than V2.

There seems to be some inefficient behaviors in the handling of
V3 protocol or in libpq. Are there any TODO items here?

----
pg_ctl start -o "-c shared_buffers=1GB"
./pgbench -n -S -s50 -c16 -t100000 -M [V2|V3|PREPARE]
transaction type: SELECT only
scaling factor: 50
number of clients: 16
number of transactions per client: 100000
number of transactions actually processed: 1600000/1600000

sql mode: V2
tps = 19025.932203 (including connections establishing)
tps = 19036.285885 (excluding connections establishing)

sql mode: V3
tps = 16699.173521 (including connections establishing)
tps = 16707.135700 (excluding connections establishing)

sql mode: PREPARE
tps = 26629.710398 (including connections establishing)
tps = 26649.901658 (excluding connections establishing)

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center