gistfile1.txt

    public class Benchmarks
    {
        static byte[] array = new byte[32];
        static int res;

        [Params(ulong.MaxValue)]
        public ulong Input { get; set; }        

        [Benchmark]
        public bool StringVersion()
        {
            return Best.StringVersion(Input, ref res);
        }

        [Benchmark]
        public bool ConditionVersion()
        {
            return Best.ConditionVersion(Input, ref res);
        }

        [Benchmark]
        public bool ROSVersion()
        {
            return Best.ROSVersion(Input, ref res);
        }
    }

    static class Best
    {
        public static bool ConditionVersion(ulong value, ref int res)
        {
            int computedOutputLength = 70;

            while (--computedOutputLength != 0)
            {
                res += (byte)Convert_Condition((int)value);
                value >>= 1;
            }

            return true;
        }

        public static bool StringVersion(ulong value, ref int res)
        {
            int computedOutputLength = 70;

            while (--computedOutputLength != 0)
            {
                res += (byte)Convert_String((int)value);
                value >>= 1;
            }

            return true;
        }

        public static bool ROSVersion(ulong value, ref int res)
        {
            int computedOutputLength = 70;

            while (--computedOutputLength != 0)
            {
                res += (byte)Convert_ROS((int)value);
                value >>= 1;
            }

            return true;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static char Convert_Condition (int value)
        {
            value &= 0xF;
            return (char)(value > 9 ? value - 10 + 'A' : value + '0');
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static char Convert_String (int value)
        {
            return "0123456789ABCDEF"[value & 0xF];
        }

        internal static ReadOnlySpan<byte> ros => new byte[16]
        {
            (byte)'0', (byte)'1', (byte)'2', (byte)'3', (byte)'4', (byte)'5', (byte)'6', (byte)'7',
            (byte)'8', (byte)'9', (byte)'A', (byte)'B', (byte)'C', (byte)'D', (byte)'E', (byte)'F'
        };

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static char Convert_ROS (int value)
        {
            return (char)ros[value & 0xF];
        }
    }

BenchmarkDotNet=v0.12.0, OS=macOS 10.15.2 (19C57) [Darwin 19.2.0]
Intel Core i7-7920HQ CPU 3.10GHz (Kaby Lake), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=5.0.100-alpha.1.20061.5
  [Host]     : .NET Core 5.0.0 (CoreCLR 5.0.20.6301, CoreFX 4.700.19.56404), X64 RyuJIT
  DefaultJob : .NET Core 5.0.0 (CoreCLR 5.0.20.6007, CoreFX 5.0.20.6007), X64 RyuJIT


|           Method |                Input |     Mean |    Error |   StdDev |
|----------------- |--------------------- |---------:|---------:|---------:|
|    StringVersion | 18446744073709551615 | 96.21 ns | 1.507 ns | 1.336 ns |
| ConditionVersion | 18446744073709551615 | 87.36 ns | 0.485 ns | 0.430 ns |
|       ROSVersion | 18446744073709551615 | 99.03 ns | 1.316 ns | 1.231 ns |

As a counter point:

BenchmarkDotNet=v0.12.0, OS=Windows 10.0.18363
AMD Ryzen 9 3900X, 1 CPU, 24 logical and 12 physical cores
.NET Core SDK=5.0.100-alpha.1.20064.2
  [Host]     : .NET Core 5.0.0 (CoreCLR 5.0.20.6307, CoreFX 5.0.20.6307), X64 RyuJIT  [AttachedDebugger]
  DefaultJob : .NET Core 5.0.0 (CoreCLR 5.0.20.6307, CoreFX 5.0.20.6307), X64 RyuJIT


|           Method |                Input |     Mean |    Error |   StdDev |
|----------------- |--------------------- |---------:|---------:|---------:|
|    StringVersion | 18446744073709551615 | 44.64 ns | 0.235 ns | 0.220 ns |
| ConditionVersion | 18446744073709551615 | 58.82 ns | 1.199 ns | 1.516 ns |
|       ROSVersion | 18446744073709551615 | 43.57 ns | 0.291 ns | 0.227 ns |

This can depend greatly on the CPU you are running against, the input payload, and a number of other factors