Slow time updating primary key indexed row

I have a query that updates a field in a table using the primary key to locate the row. The table can contain many rows where the date/time field is initially NULL, and then is updated with a date/time stamp using NOW().

When I run the update statement on the table, I am getting a slow query log entry (3.38 seconds). The log indicates that 200,000 rows were examined. Why would that many rows be examined if I am using the PK to identify the row being updated?

Primary key is item_id and customer_id. I have verified the PRIMARY key is correct in the mySQL table structure.

UPDATE cust_item
   SET status = 'approved',
       lstupd_dtm = NOW()
 WHERE customer_id = '7301'
   AND item_id = '12498';

Solution

I wonder if it's a hardware issue.

While the changes I've mentioned in comments might help slightly, in truth, I cannot replicate this issue...

I have a data set of roughly 1m rows...:

CREATE TABLE cust_item
(customer_id INT NOT NULL
,item_id INT NOT NULL
,status VARCHAR(12) NULL
,PRIMARY KEY(customer_id,item_id)
);

-- INSERT some random rows...

SELECT COUNT(*)
     , SUM(customer_id = 358) dense
     , SUM(item_id=12498) sparse 
FROM cust_item;
+----------+-------+--------+
| COUNT(*) | dense | sparse |
+----------+-------+--------+
|  1047720 |   104 |      8 |
+----------+-------+--------+

UPDATE cust_item 
   SET status = 'approved' 
 WHERE item_id = '12498' 
   AND customer_id = '358';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0