Unreliable package response time

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I´m having issues getting a reliable response time for packages sent and received in a python application. Using Pair1 to listen on a Windows 10 machine for connections from a Ubuntu machine. Packages are sent from Windows to the Ubuntu machine, where they are immediately send back as response. Response time for small packages (<58 bytes paylod) is slow (around 2-3ms) and as expected. However payloads >=58 bytes are giving an unreliable response times (3ms- 300ms) and even some packages are retransmitted. Not sure if that behaviour is related to pynng/nng or more an issue of the OS.(TCP_NODELAY is True). Do you have any idea what could cause this?