Strange increase in latency of ZeroMQ between c# and python

I'm trying to send images from C# running on win 11 to a python server running on WSL2.

I'm using ZeroMQ to send byte arrays, but as soon as the byte arrays cross a threshold (~8kb) the latency spikes from 0.3ms to 50ms. This 50ms latency remains until I get to ~74KB. Then it reduces back to about 0.3ms.

I have seen references to Nagle's algorithm (https://stackoverflow.com/a/75190133/1988046) but it seems that Nagle's is disabled by default in zeromq (http://wiki.zeromq.org/area:faq).

[Update] I ran it with a local install of python on the Win 11 side and did not see this problem at all, even though I was using exactly the same code file.

Can anyone suggest what I'm doing wrong?

C# Client

using System.Diagnostics;
using NetMQ;
using NetMQ.Sockets;

namespace ChatGptUdp;

internal class Program
{
    private static void Main(string[] args)
    {
        var socket = new RequestSocket();
        socket.Connect("tcp://localhost:5555");
        var data = new byte[100000];
        for (var i = 0; i < data.Length; i++) data[i] = (byte) i;

        for (var i = 1; i < 100; i++)
        {
            var targetLength = 1000 * i;
            var chunk = data.Take(targetLength).ToArray();
            var elapsed = new List<TimeSpan>();
            for (var j = 0; j < 100; j++)
            {
                var sw = Stopwatch.StartNew();
                socket.SendFrame(chunk);
                var response = socket.ReceiveFrameString();
                sw.Stop();
                elapsed.Add(sw.Elapsed);
            }
            // I skip the first 50 so that I ignore any startup delays. 
            // I don't think that will happen with this version of my algorithm, but just to be sure.
            Console.Out.WriteLine($"{targetLength}: {elapsed.Skip(50).Average(s => s.TotalMilliseconds)}");
        }
    }
}

Python Server

import zmq
import numpy as np
context = zmq.Context()
socket = context.socket(zmq.REP)
socket.bind('tcp://*:5555')
print("Server connected")
while True:
    msg = socket.recv()
    data = {
        'str': 'Hello there!'
    }
    socket.send_json(data)

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