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By default panels assumes that you would never want to cache any displays / panes on a $_POST request.
This is quite an assumption, in some cases you might want the contents of a pane, or an entire display to be cached on a $_POST request (for example if it will display the exact same content regardless of GET or POST).
On some pages on our site (where we have a form and surrounding mini panels) - this is giving us a boost of around 2 seconds.
Patch to follow...