Q:

How can I set time for insertAfter

How can I set time for insertAfter , more details : 
$(z).find(".post--"+a).insertAfter(".post:last");

for example when i use .slideDown(time) it will slide with the time I set now i want to know how can do that for insertAfter

A:

As I suggested in the comment, you can try
$(z).find(".post--"+a).hide().insertAfter(".post:last").slideDown(500);

