it seems client does not receive tcp ack from f5 so client does tcp retransmission.
i see at least 2 probable causes:
a. "Acknowledge on Push" is enabled in virtual server's client side tcp profile while server response latency is long.
hence, client does not receive tcp ack because f5 has not received server response to be pushed to client alongside the ack.
https://my.f5.com/manage/s/article/K50411377#datatransfer
b. F5 - client network connection is bad.
i suggest you collect tcpdump in client and f5 to see more details.