faster version of https://gist.github.com/vcsjones/7e893b22555331d86698148d39b40235

uuidv7.cs

using System;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Security.Cryptography;

public readonly struct UUIDv7 {
    private readonly InteriorStruct _interior;

    public UUIDv7() {
        InteriorStruct.Fill(ref _interior);
    }

    public byte[] GetBytes() => _interior.AsReadOnlySpan().ToArray();
    public void CopyTo(Span<byte> destination) => _interior.AsReadOnlySpan().CopyTo(destination);
    public override string ToString() => Convert.ToHexString(_interior.AsReadOnlySpan());

    public readonly override int GetHashCode() {
        HashCode hc = new();
        hc.AddBytes(_interior.AsReadOnlySpan());
        return hc.ToHashCode();
    }

    [InlineArray(SIZE)]
    private struct InteriorStruct {
        private const int SIZE = 16;
        private byte _element;

        public static void Fill(ref InteriorStruct uuid) {
            Span<byte> value = MemoryMarshal.CreateSpan(ref uuid._element, SIZE);
            RandomNumberGenerator.Fill(value[6..]);

            long timestamp = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
            var timestampBytes = MemoryMarshal.AsBytes(MemoryMarshal.CreateSpan(ref timestamp, length: 1));
            timestampBytes[..6].CopyTo(value[..6]);
            value[..6].Reverse();

            // version and variant
            value[6] = (byte)((value[6] & 0x0F) | 0x70);
            value[8] = (byte)((value[8] & 0x3F) | 0x80);
        }

        [UnscopedRef]
        internal ReadOnlySpan<byte> AsReadOnlySpan() => MemoryMarshal.CreateReadOnlySpan(ref _element, SIZE);
    }
}

A slightly faster Fill (I think):

        public static void Fill(ref InteriorStruct uuid) {
            Span<byte> value = MemoryMarshal.CreateSpan(ref uuid._element, SIZE);

            long timestamp = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds() << 16;
            BinaryPrimitives.WriteInt64BigEndian(value, timestamp);
            RandomNumberGenerator.Fill(value[6..]);

            // version and variant
            value[6] = (byte)((value[6] & 0x0F) | 0x70);
            value[8] = (byte)((value[8] & 0x3F) | 0x80);
        }