Optimizing Aggregate for String Concatenation

You are 'overriding' System.Linq.Aggregate with your own extension method in namespace MakeAggregateGoFaster.

Perhaps specialised on IEnumerable<string> and making use of a StringBuilder?

Maybe taking an Expression<Func<string, string, string>> instead of a Func<string, string, string> so it can analyse the expression tree and compile some code that uses StringBuilder instead of calling the function directly?

Just guessing.

Why not use one of the other forms of Aggregate?

Enumerable.Range(0, size ).Aggregate(new StringBuilder(),
        (a, b) => a.Append(", " + b.ToString()),
        (a) => a.Remove(0,2).ToString());

You can specify any type for your seed, perform whatever formatting or custom calls are needed in the first lambda function and then customize the output type in the second lambda function. The built in features already provide the flexibility you need. My runs went from 1444ms to 6ms.