For large numbers of registrations the export feature needs to be broken down into smaller sections to prevent the server from timing out. This function allows you to set a limit on th query, run the export to download X amount of records, then update the limit and run the export again to get the next 'section' of registrations and so on....

ee_ssp_export_limits.php

<?php
/*
Plugin Name: EE Site Specific Plugin for EE Export functions
Description: This plugin allows you to set a limit on the query used for the CSV export.
*/
/* Begin Adding Functions Below This Line; Do not include an opening PHP tag as this sample code already includes one! */
add_filter( 'FHEE__EE_Export__report_registration_for_event', 'ee_custom_export_limits', 10, 1 );
function ee_custom_export_limits( $current_query) {

    // The query limit is set here.
    $current_query['limit'] = array(0,400);
    
    //array({offset}, {limit})
    //Offest = the row to start from
    //Limit = the amount of rows to pull in, starting from the offset value.
    //So to export 1200 registrations you can break this down into:
    //array(0,400); (then run the export and save the file)
    //array(400, 400); (again run the export and save the file)
    //array(800, 400); (run the final export and save the file)
    
    return $current_query;
}

/* Stop Adding Functions */