Most application webservers (by default) handle one request per thread. For mostly IO bound stuff (which many projects are), it makes sense to me that threads become a bottleneck in relatively ordinary scenarios.
For example, this program:
var threads = new Thread[20000];
for (int i = 0; i < 20000; i++) {
threads[i] = Thread.ofVirtual().start(() -> {
try {
Files.copy(FileSystems.getDefault().getPath("abc.txt"), System.out);
} catch (IOException e) {
System.err.println("Error writing file");
e.printStackTrace();
}});
}
for (int i = 0; i < 20000; i++) {
threads[i].join();
}
2. You did not compare against fewer threads to see if threads are actually the bottleneck rather than IO. Also, all your threads are competing for stdout.