Help improve my migration performance

I've got what I thought was going to be a fairly straightforward migration of a very large data set (~10M rows of .csv supplied in 9 files) but when I run one (on a Macbook with MAMP) it took 21.5 hours processing 650 lines/second to run the first file of ~1M rows. I would really appreciate advice from anyone about where to start in terms of improving the performance. (And if I'm barking up the wrong tree and there's not much to be done, please let me know before I waste too much time fiddling and just learn to be patient)

The csv data is in a bit of a screwey format in that I have a row which contains the 'customer' data which gets turned into 'customer' nodes (that migrates fairly quickly) and 'rate analysis' data which gets turned into a field collection as part of the subsequent migration, followed by 10-14 rows of bill period data which gets migrated into a field collection. Then there's another customer & rate row and another 10-14 rows of bill period data.

So I wrote this:

function migrate_customers_migrate_api() {
  $api = array(
    'api' => 2,
    'migrations' => array(
      'CustomerData1' => array('class_name' => 'CustomerDataMigration1'),
      'RateAnalysis1' => array('class_name' => 'RateAnalysisMigration1'),
      'BillPeriod1' => array('class_name' => 'BillPeriodMigration1'),
    ),
  );
  return $api;
}

//then there's the CustomerDataMigration
//then there's the RateAnalysisMigration which works fast enough too

/**
 * Migrate the bill period data into the entity collections for each rate analysis.
 */
abstract class BillPeriodMigration extends DynamicMigration {
  public function __construct() {
    parent::__construct();
    $this->description = t('Import bill period data from CSV file.');
    //$this->softDependencies = array('CustomerData');

    // Create a map object for tracking the relationships between source rows
    $this->map = new MigrateSQLMap($this->machineName,
        array(
          'account_ID' => array('type' => 'varchar',
                           'length' => 12,
                           'not null' => TRUE,
                           'description' => 'account_ID',
                          ),
          'rate_plan_id' => array('type' => 'varchar',
                           'length' => 255,
                           'not null' => TRUE,
                           'description' => 'rate_plan_id',
                          ),
          'bill_period_start_date' => array('type' => 'varchar',
                           'length' => 255,
                           'not null' => TRUE,
                           'description' => 'bill_period_start_date',
                          ),
        ),
        MigrateDestinationNode::getKeySchema()
      );
    
    $this->destination = new MigrateDestinationFieldCollection(
      'field_bill_period', 
      array('host_entity_type' => 'node')
	);

    //$this->addFieldMapping('host_entity_id', 'account_ID')->sourceMigration('CustomerData');
    $this->addFieldMapping('field_rate_plan_id', 'rate_plan_id');
    $this->addFieldMapping('field_bill_period_cost', 'bill_period_cost');
    $this->addFieldMapping('field_bill_period_start', 'bill_period_start_date');
    $this->addFieldMapping('field_bill_period_end', 'bill_period_end_date');
    
    $this->addFieldMapping(NULL, 'account_ID');
   
    $this->addUnmigratedSources(array(
    	'analysis_date',
        'util_internal_id',
    	'customer_first_name',
    	'customer_last_name',
    	'record_type', 
    	"mailing_address_line_1",
    	"mailing_address_line_2",
    	"mailing_address_line_3",
    	"mailing_address_line_4", 
    	"service_address_line_1",
    	"service_address_line_2", 
    	"service_address_line_3",
    	"service_address_line_4",
    	'default_plan',
    	'current_plan',
    	'lowest_total_cost_plan',
    	'total_cost',
    	'total_start_date',
    	'total_end_date',
    	'total_bill_periods',
    	'has_errors'
    ));    
  }

  function prepareRow($row) {
	if (parent::prepareRow($row) === FALSE) {
	  return FALSE;
	}
  	// Don't import customer rows or glitched bill periods with no cost
  	if($row->record_type == "TOTAL" || $row->bill_period_cost == ''){
	    return FALSE;
    }
  }
}



//BILL PERIOD MIGRATIONS

class BillPeriodMigration1 extends BillPeriodMigration {
  public function __construct() {
    // Always call the parent constructor first for basic setup
    parent::__construct();
    $file = dirname(__FILE__) . '/data/'.FILE1;
    //$file = dirname(__FILE__) . '/data/2013_Feb_OPower_BRATInput_1237568810.csv';
    $this->source = new MigrateSourceCSV($file, array(), array('header_rows' => 1) );
    
    $this->addFieldMapping('host_entity_id', 'account_ID')->sourceMigration('CustomerData1');

  }
}