I am not using any third party plugin except q2a plugins.
I found the following in performance schema when the issue occurred:
LOCK TABLES qa_blobs WRITE, qa_cache WRITE, qa_categories WRITE, qa_categorymetas WRITE, qa_chat_kicks WRITE, qa_chat_posts WRITE, qa_chat_users WRITE, qa_contentwords WRITE, qa_cookies WRITE, qa_eventlog WRITE, qa_iplimits WRITE, qa_mamlmConfirm WRITE, qa_messages WRITE, qa_options WRITE, qa_pages WRITE, qa_postmetas WRITE, qa_posts WRITE, qa_posttags WRITE, qa_sharedevents WRITE, qa_tagmetas WRITE, qa_tagwords WRITE, qa_titlewords WRITE, qa_userevents WRITE, qa_userfavorites WRITE, qa_userfields WRITE, qa_userlevels WRITE, qa_userlimits WRITE, qa_userlogins WRITE, qa_usermeta WRITE, qa_usermetas WRITE, qa_usernotices WRITE, qa_userpoints WRITE, qa_userprofile WRITE, qa_users WRITE, qa_uservotes WRITE, qa_widgetanyw WRITE, qa_widgets WRITE, qa_words WRITE
due to which all read operation goes in wait status for a long duration