A simple INSERT query on InnoDB, which is too much

I have this simple query:

INSERT IGNORE INTO beststat (bestid,period,rawView) VALUES ( 4510724 , 201205 , 1 ) 

In the table:

CREATE TABLE `beststat` (
 `bestid` int(11) unsigned NOT NULL,
 `period` mediumint(8) unsigned NOT NULL,
 `view` mediumint(8) unsigned NOT NULL DEFAULT '0',
 `rawView` mediumint(8) unsigned NOT NULL DEFAULT '0',
 PRIMARY KEY (`bestid`,`period`),
) ENGINE=InnoDB AUTO_INCREMENT=2020577 DEFAULT CHARSET=utf8

And it takes 1 second to complete.

Side note: this does not actually take 1 second. Sometimes this was done even after 0.05 seconds. But often it takes 1 second

This table (beststat) currently has entries of ~ 500,000 and its size: 40 MB . I have 4 GB of RAM and innodb buffer pool size= 104 857 600 , s: Mysql: 5.1.49-3

This is the only InnoDB table in my database (others are MyISAM)

ANALYZE TABLE beststat shows: OK

Maybe something is wrong with InnoDB settings?