Migration of Users very slow with d2d
In settings.php set:


  $conf['password_count_log2'] = 1; 


You will speed up how drupal handles user passwords: The problems reported here: https://www.drupal.org/node/2200181 ... seem to match yours, first 2000 in X seconds second 2000 in X.Y seconds etc...

Also, in php.ini set:


  max_execution_time to 0 (for unlimited) or a high number like 3000 rather than the default 30.


NB. Be sure to return the execution time to its default setting after the migration.