Inserting large number of rows into database with Spring boot

I need to insert many million rows + many GB of data into a database for a project that uses Spring boot. I recreated a minimal example with a one to many relationship and am trying to find the fastest solution. The full code is here: https://github.com/Vuizur/springmassinsert, but the rough structure is:

// Email.java@Entity@Table(name = "emails")public class Email {    @Id    @GeneratedValue(strategy = GenerationType.IDENTITY)    private Long id;    @Column(name = "address")    private String address;    @Column(name="text")    private String text;    @ManyToOne(fetch = FetchType.LAZY)    @JoinColumn(name = "user_id")    private User user;}// User.java@Entity@Table(name = "users")public class User {    @Id    @GeneratedValue(strategy = GenerationType.IDENTITY)    private Long id;    @Column(name = "name")    private String name;    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL, fetch = FetchType.LAZY)    private List<Email> emails;}

But the most important part is the insert code, where I tested 4 versions:

@Servicepublic class InsertService {    @Autowired    private JdbcTemplate jdbcTemplate;    @Autowired    private UserRepository userRepository;    private final int NUMBER_OF_USERS = 10000;    private final int NUMBER_OF_EMAILS = 10;    public void insert() {        List<User> users = new ArrayList<>();        for (int i = 0; i < NUMBER_OF_USERS; i++) {            User user = new User();            user.setName("User " + i);            List<Email> emails = new ArrayList<>();            for (int j = 0; j < NUMBER_OF_EMAILS; j++) {                Email email = new Email();                email.setAddress("email" + j +"@gmail.com");                email.setUser(user);                emails.add(email);            }            user.setEmails(emails);            users.add(user);        }        userRepository.saveAll(users);    }    public void insertBatch() {        List<User> users = new ArrayList<>();        for (int i = 0; i < NUMBER_OF_USERS; i++) {            User user = new User();            user.setName("User " + i);            List<Email> emails = new ArrayList<>();            for (int j = 0; j < NUMBER_OF_EMAILS; j++) {                Email email = new Email();                email.setAddress("email" + j +"@gmail.com");                email.setUser(user);                emails.add(email);            }            user.setEmails(emails);            users.add(user);            if (i % 1000 == 0) {                userRepository.saveAll(users);                users.clear();            }        }    }    public void insertJdbc() {        List<User> users = new ArrayList<>();        for (int i = 0; i < NUMBER_OF_USERS; i++) {            User user = new User();            user.setName("User " + i);            List<Email> emails = new ArrayList<>();            for (int j = 0; j < NUMBER_OF_EMAILS; j++) {                Email email = new Email();                email.setAddress("email" + j +"@gmail.com");                email.setUser(user);                emails.add(email);            }            user.setEmails(emails);            users.add(user);        }        for (User user : users) {            jdbcTemplate.update("insert into users (name) values (?)", user.getName());            for (Email email : user.getEmails()) {                jdbcTemplate.update("insert into emails (address, text, user_id) values (?, ?, ?)", email.getAddress(),                        email.getText(), user.getId());            }        }    }    public void insertJdbcBatch() {        List<User> users = new ArrayList<>();        for (int i = 0; i < NUMBER_OF_USERS; i++) {            User user = new User();            user.setName("User " + i);            List<Email> emails = new ArrayList<>();            for (int j = 0; j < NUMBER_OF_EMAILS; j++) {                Email email = new Email();                email.setAddress("email" + j +"@gmail.com");                email.setUser(user);                emails.add(email);            }            user.setEmails(emails);            users.add(user);        }            try {            // Create a prepared statement for inserting users            PreparedStatement userPs = jdbcTemplate.getDataSource().getConnection()                    .prepareStatement("insert into users (name) values (?)", Statement.RETURN_GENERATED_KEYS);            // Create a prepared statement for inserting emails            PreparedStatement emailPs = jdbcTemplate.getDataSource().getConnection()                    .prepareStatement("insert into emails (address, text, user_id) values (?, ?, ?)");            for (User user : users) {                // Set the user name parameter and add to the batch                userPs.setString(1, user.getName());                userPs.addBatch();            }            // Execute the batch update for users and get the generated ids            userPs.executeBatch();            ResultSet rs = userPs.getGeneratedKeys();            int index = 0;            while (rs.next()) {                // Set the user id from the result set                users.get(index).setId(rs.getLong(1));                index++;            }            for (User user : users) {                for (Email email : user.getEmails()) {                    // Set the email parameters and add to the batch                    emailPs.setString(1, email.getAddress());                    emailPs.setString(2, email.getText());                    emailPs.setLong(3, user.getId());                    emailPs.addBatch();                }            }            // Execute the batch update for emails            emailPs.executeBatch();        } catch (Exception e) {            System.out.println(e);        }    }    @PostConstruct    public void benchmark() {        long startTime = System.currentTimeMillis();        insertJdbcBatch();        long endTime = System.currentTimeMillis();        System.out.println("Inserting " + NUMBER_OF_USERS +" users with " + NUMBER_OF_EMAILS +" emails each took "+ (endTime - startTime) +" milliseconds");        // Print insert per seconds        System.out                .println((NUMBER_OF_USERS * NUMBER_OF_EMAILS * 1.0 / ((endTime - startTime) / 1000.0)) +" inserts per second");    }}

The performance results are (on a bad external HDD):

So the difference between the first two versions is not significant, the naive JDBC version is really slow, but JDBC with prepared statements is the clear winner. Is there something else I can do to speed up the inserts even more?