How to break down importing of feeds

You can try using wp cron functions, programing a feed import every 10 minutes or so, until you have nothing left in queue.

Example code:

<?php
register_activation_hook( __FILE__, 'wp1903_schedule_cron' );

function wp1903_schedule_cron() {
    $counter = 0;
    $feeds = array(
        'http://example.com/feed1',
        'http://example.com/feed2',
        'http://example.com/feed3',
        'http://example.com/feed4'
    );

    foreach ($feeds as $feed) {
        $counter++;
        // schedule every 10 mins
        $time = time() + (600 * $counter);
        wp_schedule_single_event( $time, 'wp1903_process_feed', $feed );
    }
}

add_action('wp1903_process_feed', 'wp1903_import_feed');

function wp1903_import_feed($feed_url="") {
    // do the import
}

It is essential only to run the scheduler on a activation hook, or else at every page load the crons will be rescheduled.