input type="hidden" value="....a chain of rubbish letter..."!!

I use the browser view source function to see
the html generated by jsf,
and always see something like the following:

<input type="hidden" name="javax.faces.ViewState" id="javax.faces.ViewState" value="8EgC7hvJoXWgMHaUZxk5rx66APlnNueyP32ajDxbvc
/i5akMf2jX5ZGyBjrnsmcbFDLXCaS8cHN169OeG5k
DXxvWNcBMTAigJiokZI5Ne66G0/CILkEquml3xSn
+jI4+HDkEeDEBtmlPDjjFJyOcOmZ+87klHuSTgAe
5P6mMDOQOqrH0Pj9yl+
............++nBOnItypO1I+XXANTsWVuCGc
V2sghVuKD42Kt/UFL7c6gh4S7KuRSQTuzsQzs
kJZBO7OxDOyQl0Yq1NF68G0U6T4uIj27tlXp
OehNZq4LNOhvQdQH7D6ZKdmAKbVesQLrzhzo
9mnY9MN0zJj1r9NhEWfhlFpRxIxYKVda+2p
PzvUD9+wbS2NpmjNu5opnJ++XI9z5KpWbdb
lb9JKIaP6Zzxdsh2E41VBicsyuBzaOTZoz
buaKZyfvJAmuJTdCe+/H9z8iOGr8SQHtmk1
8OUgs7l4BGSVeW3JGgo6F2EHIjEMwkhmgQI
kRwDYBkboU2tAin0sPxMybTF65p98Jrn1Gs
......10bBgnfB2FWSo74DqQ==" />

the value is a chain of meaningless letter, what is it?
will it slow down the page rendering in browser?

JSF uses this hidden attribute to save the state of the view in an encrypted form. You'll find details in the JSF Specification, "7.6.2 State Saving Alternatives and Implications".