How can I increase the rate at which the 'fetch bounce' job works
Here is one approach - modify /civicrm/CRM/Utils/Mail/EmailProcessor.php

so it has multiple iterations specified (in example below = 40) then run via 

drush cvapi Job.fetch_bounces

around line 178

// process fifty at a time, CRM-4002
    $iterations = 0;
    while ($iterations &lt; 40 &amp;&amp; $mails = $store-&gt;fetchNext(MAIL_BATCH_SIZE)) {
      $iterations++;
      print("\nBeginning iteration $iterations ");
      foreach ($mails as $key =&gt; $mail) {
        print(".");