So after updating my source code it looks that it performs fast enough and allocates no memory. Here are benchmark for comparing the most popular methods of generating guids.

Runtime=.NET Core 3.0 Force=True Server=True

|                              Method |      Mean |    Error |   StdDev | Ratio | RatioSD |  Gen 0 | Gen 1 | Gen 2 | Allocated |
|------------------------------------ |----------:|---------:|---------:|------:|--------:|-------:|------:|------:|----------:|
|          SqlServerNewSequentialGuid |  38.69 ns | 0.787 ns | 1.643 ns |  1.00 |    0.00 | 0.0009 |     - |     - |      80 B |
|                       Guid_Standard |  61.44 ns | 0.665 ns | 0.622 ns |  1.60 |    0.06 |      - |     - |     - |         - |
|        NHibernate_GuidCombGenerator | 160.22 ns | 3.070 ns | 2.721 ns |  4.16 |    0.20 | 0.0012 |     - |     - |     104 B |
|                       Guid_Comb_New | 291.56 ns | 5.421 ns | 4.805 ns |  7.57 |    0.37 | 0.0010 |     - |     - |     104 B |
| EFCore_SequentialGuidValueGenerator |  82.04 ns | 1.623 ns | 1.667 ns |  2.13 |    0.11 | 0.0007 |     - |     - |      72 B |
|                           NewId_Lib |  78.93 ns | 1.150 ns | 1.076 ns |  2.05 |    0.09 |      - |     - |     - |         - |
|       NewSequentialGuid_PureNetCore |  70.28 ns | 1.332 ns | 1.481 ns |  1.81 |    0.08 |      - |     - |     - |         - |

So after updating my source code it looks that it performs fast enough and allocates no memory. Here are benchmark for comparing the most popular methods of generating GUIDs.

Runtime=.NET Core 3.0 Force=True Server=True

using System;
using System.Buffers;
using System.Linq;
using System.Net.NetworkInformation;
using System.Threading;

namespace PerfBenchmarkDotNet
{
    internal readonly struct FastGuid
    {
        private static readonly byte[] _macBytes;
        private static long _baseDateTicks;
        private static long _clockSequenceNumber;

        static FastGuid()
        {
            _clockSequenceNumber = 0;
            _macBytes = GetDefaultMacAdress();
            _baseDateTicks = new DateTime(1582, 10, 15, 0, 0, 0, DateTimeKind.Utc).Ticks;
        }

        public static Guid NewGuid()
        {
            var nowTicks = DateTime.UtcNow.Ticks;
            var sequenceNumber = Interlocked.Increment(ref _clockSequenceNumber);
            var ticksDiff = nowTicks - _baseDateTicks;

            var a = (int)(ticksDiff);
            short b = (short)(ticksDiff >> 32);
            short c = (short)(ticksDiff >> 48);

            return new Guid(
                a,
                b,
                c,
                (byte)(sequenceNumber >> 8),
                (byte)(sequenceNumber),
                _macBytes[0],
                _macBytes[1],
                _macBytes[2],
                _macBytes[3],
                _macBytes[4],
                _macBytes[5]);
        }

        private static byte[] GetDefaultMacAdress()
        {
            var nic = NetworkInterface.GetAllNetworkInterfaces().FirstOrDefault();

            if (nic != null)
            {
                return nic.GetPhysicalAddress().GetAddressBytes();
            }

            var fallback = Guid.NewGuid().ToByteArray();
            return fallback.Take(6).ToArray();
        }
    }
}

|                              Method |      Mean |    Error |   StdDev | Ratio | RatioSD |  Gen 0 | Gen 1 | Gen 2 | Allocated |
|------------------------------------ |----------:|---------:|---------:|------:|--------:|-------:|------:|------:|----------:|
|          SqlServerNewSequentialGuid |  38.69 ns | 0.787 ns | 1.643 ns |  1.00 |    0.00 | 0.0009 |     - |     - |      80 B |
|                       Guid_Standard |  61.44 ns | 0.665 ns | 0.622 ns |  1.60 |    0.06 |      - |     - |     - |         - |
|        NHibernate_GuidCombGenerator | 160.22 ns | 3.070 ns | 2.721 ns |  4.16 |    0.20 | 0.0012 |     - |     - |     104 B |
|                       Guid_Comb_New | 291.56 ns | 5.421 ns | 4.805 ns |  7.57 |    0.37 | 0.0010 |     - |     - |     104 B |
| EFCore_SequentialGuidValueGenerator |  82.04 ns | 1.623 ns | 1.667 ns |  2.13 |    0.11 | 0.0007 |     - |     - |      72 B |
|                           NewId_Lib |  78.93 ns | 1.150 ns | 1.076 ns |  2.05 |    0.09 |      - |     - |     - |         - |
|       NewSequentialGuid_PureNetCore |  70.28 ns | 1.332 ns | 1.481 ns |  1.81 |    0.08 |      - |     - |     - |         - |

using System;
using System.Buffers;
using System.Linq;
using System.Net.NetworkInformation;
using System.Threading;

namespace PerfBenchmarkDotNet
{
    internal readonly struct FastGuid
    {
        private static readonly byte[] _macBytes;
        private static long _baseDateTicks;
        private static long _clockSequenceNumber;

        static FastGuid()
        {
            _clockSequenceNumber = 0;
            _macBytes = GetDefaultMacAdress();
            _baseDateTicks = new DateTime(1582, 10, 15, 0, 0, 0, DateTimeKind.Utc).Ticks;
        }

        public static Guid NewGuid()
        {
            var nowTicks = DateTime.UtcNow.Ticks;
            var sequenceNumber = Interlocked.Increment(ref _clockSequenceNumber);
            var ticksDiff = nowTicks - _baseDateTicks;

            var a = (int)(ticksDiff);
            short b = (short)(ticksDiff >> 32);
            short c = (short)(ticksDiff >> 48);

            return new Guid(
                a,
                b,
                c,
                (byte)(sequenceNumber >> 8),
                (byte)(sequenceNumber),
                _macBytes[0],
                _macBytes[1],
                _macBytes[2],
                _macBytes[3],
                _macBytes[4],
                _macBytes[5]);
        }

        private static byte[] GetDefaultMacAdress()
        {
            var nic = NetworkInterface.GetAllNetworkInterfaces().FirstOrDefault();

            if (nic != null)
            {
                return nic.GetPhysicalAddress().GetAddressBytes();
            }

            var fallback = Guid.NewGuid().ToByteArray();
            return fallback.Take(6).ToArray();
        }
    }
}