I need a slow C# function

Try to calculate nth prime number to simulate CPU intensive work -

public void Slow()
{
    long nthPrime = FindPrimeNumber(1000); //set higher value for more time
}

public long FindPrimeNumber(int n)
{
    int count=0;
    long a = 2;
    while(count<n)
    {
        long b = 2;
        int prime = 1;// to check if found a prime
        while(b * b <= a)
        {
            if(a % b == 0)
            {
                prime = 0;
                break;
            }
            b++;
        }
        if(prime > 0)
        {
            count++;
        }
        a++;
    }
    return (--a);
}

How much time it will take will depend on the hardware configuration of the system.

So try with input as 1000 then either increase input value or decrease it.

This function will simulate CPU intensive work.

Arguably the simplest such function is this:

public void Slow()
{
    var end = DateTime.Now + TimeSpan.FromSeconds(10);
    while (DateTime.Now < end)
           /* nothing here */ ;
}

You can use a 'while' loop to make the CPU busy.

    void CpuIntensive()
    {
        var startDt = DateTime.Now;

        while (true)
        {
            if ((DateTime.Now - startDt).TotalSeconds >= 10)
                break;
        }
    }

This method will stay in the while loop for 10 seconds. Also, if you run this method in multiple threads, you can make all CPU cores busy.