NewSequentialId - pure net core 2.2 implementation of sql server function
You want to reduce both computation time and memory allocation. Memory allocation itself takes time, so that's a good place to start -- try to find all the places where you're allocating memory and see if they can be avoided.

ArrayPool&lt;byte&gt;.Shared.Rent(16); can avoid an allocation, but not always. Can we do better? Turns out we can; we'll see how.

BitConverter.GetBytes(ticksDiff) makes an allocation. ticksDiff is just an integer so it's quite easy to extract its binary representation using binary operators. We'll see how we can use this later on.

But first:

Profiling

Profilers are your friend! You should always start the optimization journey by asking "Where are the bottlenecks?", which you can promptly answer with cold hard data from a profiling tool.

As suspected, dotTrace reports that the ArrayPool&lt;byte&gt;.Shared.Rent/Return and BitConverter.GetBytes(ticksDiff) calls take a significant chunk (~21%) of the total running time. Let's get rid of those.

Guid constructor

You're using the Guid constructor that takes a byte[], which has perpetuated an implementation that necessarily allocates a byte array somewhere. You can get clever with this to allocate on the stack instead of the heap (Span&lt;byte&gt; with stackalloc) or to minimize the number of allocations that actually need to happen (ThreadLocal&lt;byte[]&gt;), but the best solution is to not allocate anything at all. To accomplish this, you can use one of the other Guid constructors that takes its values directly, instead of taking a byte array.

There's a constructor overload that takes an int, two shorts, and eight bytes. You can pack these values directly (see the reference source for endianness on the int and shorts):

int a = ticksDiffBytes[7] | (ticksDiffBytes[6] &lt;&lt; 8) | (ticksDiffBytes[5] &lt;&lt; 16) | (ticksDiffBytes[4] &lt;&lt; 24);
short b = (short)(ticksDiffBytes[3] | (ticksDiffBytes[2] &lt;&lt; 8));
short c = (short)(ticksDiffBytes[1] | (ticksDiffBytes[0] &lt;&lt; 8));

var guid = new Guid(
    a,
    b,
    c,
    (byte)(sequenceNumber &gt;&gt; 8),
    (byte)(sequenceNumber),
    macBytes[0],
    macBytes[1],
    macBytes[2],
    macBytes[3],
    macBytes[4],
    macBytes[5]);


Binary stuff

You can extract bits directly from ticksDiff to avoid the call to BitConverter.GetBytes:

var ticksDiff = nowTicks - baseDateTicks;

int a = (int)(
    ((ticksDiff &gt;&gt; 56) &amp; 0xFF) |
    ((ticksDiff &amp; 0x00FF000000000000) &gt;&gt; 40) |
    ((ticksDiff &amp; 0x0000FF0000000000) &gt;&gt; 24) |
    ((ticksDiff &amp; 0x000000FF00000000) &gt;&gt; 8));

short b = (short)(
    ((ticksDiff &amp; 0x00000000FF000000) &gt;&gt; 24) |
    ((ticksDiff &amp; 0x0000000000FF0000) &gt;&gt; 8));

short c = (short)(
    ((ticksDiff &amp; 0x000000000000FF00) &gt;&gt; 8) |
    ((ticksDiff &amp; 0x00000000000000FF) &lt;&lt; 8));


If you can tolerate a change in endianness, the following is a lot cleaner to read (but provides no measurable change in runtime for me): 

var ticksDiff = nowTicks - baseDateTicks;

int a = (int)(ticksDiff);
short b = (short)(ticksDiff &gt;&gt; 32);
short c = (short)(ticksDiff &gt;&gt; 48);


Remainder

You don't need to do a remainder when computing sequenceNumber because we're extracting the remainder bits directly anyhow when we pass (byte)(sequenceNumber &gt;&gt; 8), (byte)(sequenceNumber) to the Guid constructor.

My numbers below make it hard to conclude if this helps at all. This is the kind of thing that might be optimized away by the jitter anyway.

Benchmarks

With these changes, my benchmarks look as follows:

|                                    Method |     Mean |   Error |  StdDev | Ratio |  Gen 0 | Gen 1 | Gen 2 | Allocated |
|------------------------------------------ |---------:|--------:|--------:|------:|-------:|------:|------:|----------:|
|                          NewGuid_Original | 224.2 ns | 4.37 ns | 4.49 ns |  1.00 | 0.0041 |     - |     - |      20 B |
|                  NewGuid_WithPackedValues | 153.6 ns | 2.51 ns | 2.35 ns |  1.00 | 0.0041 |     - |     - |      20 B |
| NewGuid_WithPackedValuesAndNoBitConverter | 148.6 ns | 2.43 ns | 2.28 ns |  1.00 |      - |     - |     - |         - |
|                NewGuid_WithReorderedBytes | 148.9 ns | 2.92 ns | 3.00 ns |  1.00 |      - |     - |     - |         - |
|  NewGuid_WithReorderedBytesAndNoRemainder | 146.4 ns | 2.11 ns | 1.98 ns |  1.00 |      - |     - |     - |         - |


Final code:

public static Guid NewGuid_WithBinaryOptimizationsAndWithoutRemainder()
{
    var nowTicks = DateTime.UtcNow.Ticks;
    var sequenceNumber = Interlocked.Increment(ref ClockSequenceNumber);

    var ticksDiff = nowTicks - baseDateTicks;

    int a = (int)(ticksDiff);
    short b = (short)(ticksDiff &gt;&gt; 32);
    short c = (short)(ticksDiff &gt;&gt; 48);

    var guid = new Guid(
        a,
        b,
        c,
        (byte)(sequenceNumber &gt;&gt; 8),
        (byte)(sequenceNumber),
        macBytes[0],
        macBytes[1],
        macBytes[2],
        macBytes[3],
        macBytes[4],
        macBytes[5]);

    return guid;
}