A good way to create, unload, replace, load a world

I'm creating a skywars plugin, with self forming arenas that create worlds when ever needed.
This is what I use to create, unload, replace and load a world:

Code:

final World w =Bukkit.createWorld(new WorldCreator(worldname));
                    new BukkitRunnable(){public void run() {
                        if(Bukkit.unloadWorld(w,false))
                        WorldReplicator.move(worldmodule, w.getWorldFolder());
                    }}.runTaskAsynchronously(pl);
                    Bukkit.createWorld(new WorldCreator(worldname));
                    World we = Bukkit.getWorld(worldname);
                    ArenaInstance is = new ArenaInstance(u,we,pl,m);
                    waiting.add(is);
                    long t = System.currentTimeMillis()-time;

WorldReplicator:

Code:

if (!ignore.contains(source.getName())) {
            if (source.isDirectory()) {
                if (!target.exists())
                    target.mkdirs();
                String files[] = source.list();
                for (String file : files) {
                    File srcFile = new File(source, file);
                    File destFile = new File(target, file);
                    move(srcFile, destFile);
                }
            } else {
                InputStream in = new FileInputStream(source);
                OutputStream out = new FileOutputStream(target);
                byte[] buffer = new byte[1024];
                int lenght;
                while((lenght = in.read(buffer))>0)
                    out.write(buffer,0,lenght);
                in.close();
                out.close();
            }
        }

This takes around 3.5 seconds(During that time the server breaks), and that is too much, any1 knows a way to create worlds async? as this is the only solution that I see.
Thank you for reading!

 

Pre-generate arenas and worlds, then copy them over from a separate file folder and load them.

 

Update: Setting the worldtype to flat sets the load time to 435 ms(Didn't remember that )

Already done

EDIT by Timtower: merged posts