A simple benchmark to compare performance of String.join() and String concatenation for 2 arguments.

StringJoinBenchmark.java

public class StringJoinBenchmark {
    
    public static void main(String[] args) {
        final int MAX_ITER = 1000 * 1000;
        final int RUNS = 1;

        long delta = 0;
        long delta2 = 0;

        for(int run = 0; run < RUNS; run++) {
            long start = System.nanoTime();
            for(int index = 0; index < MAX_ITER; index++) {
                String merged1 = String.join("-", "prefix", "suffix");
            }
            long end = System.nanoTime();
            
            long start2 = System.nanoTime();
            for(int index = 0; index < MAX_ITER; index++) {
                String merged2 = "prefix" + "-" + "suffix";
            }
            long end2 = System.nanoTime();
            
            delta += (end - start);
            delta2 += (end2 - start2);
        }
        
        double ratio = (double) delta / (double) delta2;
        System.out.println("Ratio: " + ratio);
    }

}

Multiple single runs provide speed improvement ratio's as:

Ratio: 102.10164665754219
Ratio: 96.11536413565226
Ratio: 101.20484979369964
Ratio: 103.62282307180902
Ratio: 102.89166083318867