SSL client connections hang and a aborted due to inactivity

Test setup:

ATS in reverse proxy mode. Terminating SSL for clients, remap to non-SSL for origin. Origin delays 10s before sending any data (including headers), then dumps a non-chunked response (Content-Length) of about 3Mbytes.

Over slow client connections, the client session freezes mid download (around 256k in my tests)... hangs until the inactivity timer fires and snips all the connections.