Strange performance issue

I am running a capacity test just for fun, and notice that if I run
one update the system uses 100% of the CPU, but if I use 2 updates at
the same time the system is only about 3% loaded and the postgres
backends rotate between
     select
     RUN
     semwait

...this is on Freebsd.

The test was just to store 100,000 new values in a single transaction...

the table definition looks like

--
-- Selected TOC Entries:
--
\connect - mvh
--
-- TOC Entry ID 2 (OID 21452)
--
-- Name: boogers Type: TABLE Owner: mvh
--

CREATE TABLE "boogers" (
    "num" integer,
    "name" character varying(20),
    "id" serial,
    "modtime" timestamp with time zone DEFAULT now()
);

--
-- Data for TOC Entry ID 5 (OID 21452)
--
-- Name: boogers Type: TABLE DATA Owner: mvh
--


COPY "boogers"  FROM stdin;
\.
--
-- TOC Entry ID 3 (OID 21452)
--
-- Name: "boogers_name" Type: INDEX Owner: mvh
--

CREATE  INDEX "boogers_name" on "boogers" using btree ( "name" "varchar_ops" );

--
-- TOC Entry ID 4 (OID 21452)
--
-- Name: "boogers_num" Type: INDEX Owner: mvh
--

CREATE  INDEX "boogers_num" on "boogers" using btree ( "num" "int4_ops" );

... and the perl code looks like

#!/usr/bin/perl

use DBI;

$dbh = DBI->connect("dbi:Pg:dbname=mvh","","",{AutoCommit => 0});

$sth = $dbh->prepare("INSERT INTO boogers VALUES (?,?)");

# insert a bunch of values
for( $i=0; $i< 100000; $i++) {
    $sth->execute( $i, $i);
}

$dbh->commit;

$rc = $dbh->disconnect;

print $rc, "\n";