If you are victimized by misterious delays in your TCP traffic, especially the light type of traffic, even in the local traffic: consider the Nagle algorithm. In this case, the observed delays are 40 ms.

A (long) time ago the kernel had a parameter to turn Nagle off globally: /proc/sys/net/ipv4/tcp_low_latency. This dissapeared with 4.14 and later versions. If you upgraded the kernel, you might start experiencing misterious delays. The only way to wrestle Nagle now is per socket: setsockopt(TCP_NODELAY)