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Trying to understand why RTT changes during a call

Hello guys.

I'm trying to use Wireshark to debug a web service call that is taking more than one second. Looking at the package list, it's possible to see that the RTT increases during the TCP conversation required for getting the ACK of one of the packages. Since I'm new, I can't upload images, so I'm dumping the 2 entries here:

116 3.854565   TLSv1.2 470 Application Data
119 4.907522    TCP 1514    443 → 34588 [ACK] Seq=5359 Ack=1048 Win=64384 Len=1448 TSval=1807772277 TSecr=1446266633 [TCP segment of a reassembled PDU]

I'm not really a network admin and my TCP/IP knowledge is limited, so I was wondering if anyone could give me any hints on why the RTT goes up from ms to more than 1 second. Any clues on what's going on here? Btw, these traces were capture between 2 Ubuntus which are on the same network.