The most likely issue is that you have session state enabled. Even if your method never touches the session state, if it's enabled for the request handler
(the page), all requests to that handler with the same session ID will be serialized.
If possible, turn off session state for the page:
<%@ Page ... EnableSessionState="False" %>
If you need to read data from the session, you might be able to get away with making it read-only:
<%@ Page ... EnableSessionState="ReadOnly" %>
If other parts of your page need to write to the session, then you'll need to move the method to a separate handler, and disable session state for that handler.
If your method needs to write data to the session, then there's nothing you can do.
ASP.NET Session State Overview | Microsoft Docs[
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