How to improve (very poor) file read performance with C# Core on an Azure Kubernetes PVC

Question

After having done a number of tests, I've narrowed the performance problem down to a combination of C# and PVC.

Problem: Read a large(ish) file (140MB) using C# Core from a PVC. Here's the (simple) test program:

using System;
using System.IO;

namespace cmaApp
{
    class Program
    {
            private static void readStream(String filePath) {
                Console.WriteLine($"Reading {filePath}");
                using (var sr = new StreamReader(new BufferedStream(new FileStream(filePath, FileMode.Open))))
                {
                        int len = sr.ReadToEnd().Length;
                        Console.WriteLine($"dat len={len}");
                }
            }

            private static void copyFile(String filePath) {
                    File.Copy(filePath, "./t2.dat");
            }

        static void Main(string[] args)
        {
            Console.WriteLine("Start!");
            String filePath = "/uploads/3864.dat";    // PVC file
            //String filePath = "./3864.dat";         // Local file

            readStream(filePath);
            //copyFile(filePath);

            Console.WriteLine("Done!");
        }
    }
}

Here's what I did, on the command line in the pod:

Test 1. Read the file with C# directly from the PVC (takes 2 mins!!):

/cmaApp# time dotnet run
Start!
Reading /uploads/3864.dat
dat len=153508096
Done!

real    2m2.172s
user    0m3.437s
sys     0m3.357s

Test 2. Copy the file to a local folder in the pod and read with C# (take 3 seconds):

cmaApp# time dotnet run
Start!
Reading ./3864.dat
dat len=153508096
Done!

real    0m2.765s
user    0m2.910s
sys     0m0.762s

Test 3. Read the file with cat directly from the PVC (takes 4 secs):

cmaApp# time cat /uploads/3864.dat |wc
 144187 5543913 153508096

real    0m4.615s
user    0m0.755s
sys     0m0.293s

Test 4. Copy the file to a local folder using cp (takes 4 secs):

cmaApp# time cp /uploads/3864.dat .

real    0m4.358s
user    0m0.000s
sys     0m0.213s

Test 5. Copy the file to a local folder using C# File.Copy (takes 13 secs):

cmaApp# time dotnet run
Start!
Done!

real    0m13.042s
user    0m2.687s
sys     0m0.913s

Seeing that the linux level cp copies the file fast, it's not a throttling / network issue.

Why is C# Core so slow? What can be done to improve it?