Very slow DELETE on 4000 rows of 55000 row table

I've got a DELETE FROM that seems to run forever, pegging the CPU at
100%.  I can't figure out why it's slow.  Any clues?


stage=# EXPLAIN DELETE FROM EG_INVOICE WHERE PERIOD_ID = 1017506;
Index Scan using ix22f7bc70c7de2059 on eg_invoice  (cost=0.00..105.39
rows=3955 width=6)  Index Cond: (period_id = 1017506)

stage=# select count(*) FROM EG_INVOICE;
55376

stage=# select count(*) FROM EG_INVOICE where PERIOD_ID = 1017506;; 4603

stage=# \d EG_INVOICE;             Table "public.eg_invoice"    Column     |          Type          | Modifiers
----------------+------------------------+-----------invoice_id     | integer                | not nullcso_id         |
integer               | not nullperiod_id      | integer                | not nullinvoice_number | character
varying(192)|invoice_date   | date                   |plan_name      | character varying(128) |invoice_style  | integer
              | not nullaccount_id     | integer                |
 
Indexes:   "eg_invoice_pkey" PRIMARY KEY, btree (invoice_id)   "invoice_number_idx" btree (invoice_number)
"ix22f7bc70c7de2059"btree (period_id)
 
Foreign-key constraints:   "fk22f7bc70c7de2059" FOREIGN KEY (period_id) REFERENCES
eg_billing_period(period_id)   "invoice_to_account" FOREIGN KEY (account_id) REFERENCES
eg_account(account_id)   "invoice_to_cso" FOREIGN KEY (cso_id) REFERENCES eg_cso(cso_id)

stage=# vacuum analyze verbose EG_INVOICE;
...
INFO:  "eg_invoice": scanned 584 of 584 pages, containing 55376 live
rows and 0 dead rows; 3000 rows in sample, 55376 estimated total rows

PostgreSQL 8.1.8 on i686-redhat-linux-gnu, compiled by GCC gcc (GCC)
4.1.1 20070105 (Red Hat 4.1.1-51)


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