[MemoryDiagnoser]
public partial class Test
{
    [GlobalSetup]
    public void Startup()
    {
    }

    string a = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA";
    byte[] bytes = new byte[1024];

    [Benchmark]
    public void UTF8()
    {
        for (int i = 0; i < 100; i++)
            Encoding.UTF8.GetBytes(a).CopyTo(bytes, 0);
    }


    [Benchmark]
    public void UTF8New()
    {
        for (int i = 0; i < 100; i++)
            a.AsSpan().ToUTF8Bytes(bytes);
    }

    [Benchmark]
    public void UTF16()
    {
        for (int i = 0; i < 100; i++)
            a.GetUTF16Bytes().CopyTo(bytes);

    }
}

BenchmarkDotNet=v0.13.5, OS=Windows 10 (10.0.19045.3208/22H2/2022Update)
AMD Ryzen 3 2200G with Radeon Vega Graphics, 1 CPU, 4 logical and 4 physical cores
.NET SDK=7.0.100
  [Host]     : .NET 7.0.0 (7.0.22.51805), X64 RyuJIT AVX2 [AttachedDebugger]
  DefaultJob : .NET 7.0.0 (7.0.22.51805), X64 RyuJIT AVX2


|  Method |       Mean |     Error |    StdDev |   Gen0 | Allocated |
|-------- |-----------:|----------:|----------:|-------:|----------:|
|    UTF8 | 5,256.1 ns | 102.87 ns | 133.77 ns | 4.2038 |    8800 B |
| UTF8New | 2,275.1 ns |  27.77 ns |  25.98 ns |      - |         - |
|   UTF16 |   586.7 ns |   5.16 ns |   4.57 ns |      - |         - |

public static int ToUTF8Bytes(this ReadOnlySpan<char> str, Memory<byte> bytes)
{
    if (str.Length == 0) return 0;
    Utf8.FromUtf16(str, bytes.Span, out var _, out var utf8Length, replaceInvalidSequences: false);
    return utf8Length;
}
public static ReadOnlySpan<byte> GetUTF16Bytes(this string str)
{
    if (str == null) return Helper.EmptyArray;
    return MemoryMarshal.AsBytes(str.AsSpan());
}