Can anybody explain what’s going on with my iteration of a list of FileStores?

I’ve got a Runnable task that’s executing once a second and calls this code:

private ArrayList<FileStore> getFileStores() {

        Iterable<FileStore> stores;
        try {
            stores = app.fileSystem.getFileStores();
        } catch (Exception ex) {
            return null;
        }

        ArrayList<FileStore> newstores = new ArrayList<>();

        long start = System.currentTimeMillis();

        if (stores != null) {
            Iterator<FileStore> iter = stores.iterator();
            while (iter.hasNext()) {
                newstores.add(iter.next());
            }
        }

        System.out.println("<<<<<<<< FILESTORE update in " + toElapsed(start));

        return newstores;
    }

And here’s the output:

<<<<<<<< FILESTORE update in 1 ms.
<<<<<<<< FILESTORE update in 144 ms.
<<<<<<<< FILESTORE update in 1 ms.
<<<<<<<< FILESTORE update in 2 ms.
<<<<<<<< FILESTORE update in 147 ms.
<<<<<<<< FILESTORE update in 2 ms.
<<<<<<<< FILESTORE update in 2 ms.
<<<<<<<< FILESTORE update in 165 ms.
<<<<<<<< FILESTORE update in 4 ms.
<<<<<<<< FILESTORE update in 134 ms.
<<<<<<<< FILESTORE update in 138 ms.
<<<<<<<< FILESTORE update in 1 ms.
<<<<<<<< FILESTORE update in 2 ms.

Why such a big difference in performance on each pass?