MiniRandomSequence.cs

struct MiniRandomSequence
{
    public MiniRandomSequence(byte max, Random rng)
    {
        if (max > 64)
            throw new ArgumentOutOfRangeException($"MiniRandomSequence max range is 63 (got {max})");

        _rng = rng;
        _max = max;
        _seen = 0;
        _done = max < 64 ? ((1L << max) - 1) : -1L;
        _value = 0; //
    }

    byte _max;
    byte _value;
    long _seen;
    readonly long _done;
    readonly Random _rng;


    public MiniRandomSequence GetEnumerator()
    {
        _seen = 0;
        return this;
    }

    public int Current => _value;

    public bool MoveNext()
    {
        if (_seen == _done)
            return false;

        while (true)
        {
            _value = (byte) _rng.Next(_max);
            if ((_seen & (1L << _value)) != 0)
                continue;

            _seen |= 1L << _value;
            return true;
        }
    }
}

on my machine (i7 7700) worst case averages out to about 17 ticks for max of 64