Hey,
When serving larger files (like 250KB) i got problem with triggering wdt in async_tcp task. After disabling it website still failed to load.
After some research I found source of the problem - too small size of _async_queue. Changing it to 256 or 512 solves my issue.

static inline bool _init_async_event_queue(){
    if(!_async_queue){
        _async_queue = xQueueCreate(512, sizeof(lwip_event_packet_t *));
        if(!_async_queue){
            return false;
        }
    }
    return true;
}

It worked for me.