In current (6.3.1) version of FOSElasticaBundle, the ORMAdapter creates a $pager using the following code:
$pager = new PagerfantaPager(new Pagerfanta(new QueryAdapter($qb)));
Not using extra parameters of QueryAdapter constructor not allows us to disable the use of output walkers.
Why I try to disable the use of output walkers?
With : when PagerFanta paginates and computes the number of results, it uses subqueries (it is a native behaviour of Doctrine)
Without output walkers : it makes a simple SELECT COUNT(*) (3x times faster)