#include <Arduino.h>
...
#include "FS.h"
#include "SD.h"
...
#include "WiFi.h"
#include "ESPAsyncWebServer.h"
#include "WebSocketsServer.h"

AsyncWebServer server(80);
...

size_t load_data(File f, uint8_t *buffer, size_t maxLen, size_t index){
  if (f.available()){
    // return f.read(buffer, maxLen);     // WDT
    return f.read();                      // extremely slow (6KB/s)
  } else {
    return 0;
  }
}

void onPageNotFound(AsyncWebServerRequest *request){
  IPAddress ip = request -> client() -> remoteIP();
  String requested_file = request->url();
  Serial.println("["+ip.toString() +"] GET Request of " + requested_file);

  if (requested_file.startsWith(folder) & SD.open(requested_file, FILE_READ)){
    File f = SD.open(requested_file, FILE_READ);
    AsyncWebServerResponse *response = request->beginChunkedResponse("text/csv", [f](uint8_t *buffer, size_t maxLen, size_t, index) -> size_t {
      return load_data(f, buffer, maxLen, index);
    });
    response->addHeader("Server","ESP Async Web Server");
    request->send(response);
    // f.close();                        // <-- don't know where to put this, it interrupts the transmission at this point
  } else {
    request->send_P(404, "text/html", "Not Found");
  }
}

void setup(){
...
 WiFi.softAP(ssid, password);
...
 server.on("/", HTTP_GET, onIndexRequest);
 server.onNotFound(onPageNotFound);
 server.begin();
}
...

E (54572) task_wdt: Task watchdog got triggered. The following tasks did not reset the watchdog in time:
E (54572) task_wdt:  - async_tcp (CPU 0/1)
E (54572) task_wdt: Tasks currently running:
E (54572) task_wdt: CPU 0: IDLE0
E (54572) task_wdt: CPU 1: loopTask
E (54572) task_wdt: Aborting.
abort() was called at PC 0x400e9e17 on core 0