ACF repeater field usage

Simply use get_sub_field("session_speaker"); function to retrieve that data.
As you’ve said this is a relational field, it will return an array of post objects according to the docs.

In order to get data from each speaker, you need to loop through the resulting array.
Example:

$speakers = get_sub_field("session_speaker");
if ($speakers && count($speakers)>0)
{
    foreach ($speakers as $speaker)
    {
        echo $speaker->ID; //$speaker is a post object//
    }        
}