List<Account> accountList = new List<Account>();

Datetime startDttm = System.now();
for( Integer i = 0; i < 1000; i++ ) {
	update accountList;
}
system.debug( 'update no checks -> ' + ( System.now().getTime() - startDttm.getTime() ) );

startDttm = System.now();
for( Integer j = 0; j < 1000; j++ ) {
    if( ! accountList.isEmpty() ) {
        update accountList;
    }
}
system.debug( 'check before update -> ' + ( System.now().getTime() - startDttm.getTime() ) );