Simple benchmark to compare StringBuilder vs string.Join

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19043.1165 (21H1/May2021Update)

Intel Core i7-9850H CPU 2.60GHz, 1 CPU, 12 logical and 6 physical cores

.NET SDK=6.0.300

[Host] : .NET Core 3.1.25 (CoreCLR 4.700.22.21202, CoreFX 4.700.22.21303), X64 RyuJIT

DefaultJob : .NET Core 3.1.25 (CoreCLR 4.700.22.21202, CoreFX 4.700.22.21303), X64 RyuJIT

Array of 26 strings

Method	Mean	Error	StdDev	Gen 0	Allocated
JoinStringTest	498.3 ns	1.70 ns	1.51 ns	0.0429	272 B
StringBuilderTest	397.0 ns	1.99 ns	1.77 ns	0.1221	768 B

StringBuilder is ~20% faster but allocates almost +200% memory with respect to string.Join

Array of 4 strings

Method	Mean	Error	StdDev	Gen 0	Allocated
JoinStringTest	138.83 ns	1.980 ns	1.852 ns	0.0153	96 B
StringBuilderTest	66.91 ns	0.256 ns	0.214 ns	0.0242	152 B

string.Join performances become worse when concatenating fewer strings