I have tried to dig further and it seems like there is some issues with Joomla 3.7 where it is performing sanitization on the post request. For some reasons, when it is filtering the POST data, it is taking ages.
I am not too sure if this is related to your php version but on my local instance it seems to be much faster. I am guessing that this could also be the same issue that the JSN guys were facing if they are using the API from Joomla.
I have replaced the codes in /components/com_easyblog/controllers/posts.php at line 803 from the following,
$data = JRequest::get('post');
To,
$data = JRequest::get('post', JREQUEST_ALLOWRAW);
It seems to be much much faster right now. Could you give this a try? However, I am not too sure if this would be an ideal fix if it doesn't goes through proper sanitization. Is there a way you could upgrade to PHP7 to test this again?