runtime: ValueStringBuilder is slower at appending short strings than StringBuilder

Investigating why my PR dotnet/corefx#27250 makes HtmlEncode slower, I wrote a quick benchmark (using BenchmarkDotNet) to compare StringBuilder and ValueStringBuilder:

[GroupBenchmarksBy(BenchmarkLogicalGroupRule.ByCategory)]
[CategoriesColumn]
public class StringBuilderBench
{
    [Benchmark(Baseline = true), BenchmarkCategory("AppendChar")]
    public void StringBuilder_AppendChar()
    {
        var builder = new StringBuilder();
        for (var i = 0; i < 100000; i++)
        {
            builder.Append('<');
        }
        builder.ToString();
        builder.Clear();
    }
    [Benchmark, BenchmarkCategory("AppendChar")]
    public void ValueStringBuilder_AppendChar()
    {
        var builder = new ValueStringBuilder();
        for (var i = 0; i < 100000; i++)
        {
            builder.Append('<');
        }
        builder.ToString();
        builder.Dispose();
    }

    [Benchmark(Baseline = true), BenchmarkCategory("AppendString")]
    public void StringBuilder_AppendString()
    {
        var builder = new StringBuilder();
        for (var i = 0; i < 100000; i++)
        {
            builder.Append("&lt;");
        }
        builder.ToString();
        builder.Clear();
    }
    [Benchmark, BenchmarkCategory("AppendString")]
    public void ValueStringBuilder_AppendString()
    {
        var builder = new ValueStringBuilder();
        for (var i = 0; i < 100000; i++)
        {
            builder.Append("&lt;");
        }
        builder.ToString();
        builder.Dispose();
    }
}

Here are the results:

                          Method |   Categories |       Mean |     Error |    StdDev | Scaled | ScaledSD |
-------------------------------- |------------- |-----------:|----------:|----------:|-------:|---------:|
        StringBuilder_AppendChar |   AppendChar |   430.9 us |  4.072 us |  3.610 us |   1.00 |     0.00 |
   ValueStringBuilder_AppendChar |   AppendChar |   359.0 us |  5.030 us |  4.459 us |   0.83 |     0.01 |
                                 |              |            |           |           |        |          |
      StringBuilder_AppendString | AppendString | 1,518.1 us | 17.672 us | 16.530 us |   1.00 |     0.00 |
 ValueStringBuilder_AppendString | AppendString | 2,230.0 us | 31.823 us | 29.767 us |   1.47 |     0.02 |

(This is using .NET Core 2.1 RC1 on OSX.) So ValueStringBuilder is faster at appending single characters but slower at appending strings than StringBuilder.