我有一个包含两个字段的简单实体

@Entity
@Table(name = "lctn_locations")
public class LocationEntity {

    @Id
    @Column(name = "id", updatable = false, nullable = false)
    private UUID id;

    @ElementCollection
    @CollectionTable(
            name = "lctn_location_devices",
            joinColumns = @JoinColumn(name = "location_id", referencedColumnName = "id")
    )
    @Column(name = "device_id")
    private Set<UUID> devices = new LinkedHashSet<>();

lctn_locations表包含780个条目。lctn_location_devices表仅包含11个条目。

我获取20个位置并将实体类转换为DTO，如下所示：

public Location map(LocationEntity locationEntity) {

     Location location = new Location();
     location.setId(locationEntity.getId());
     long c2 = System.currentTimeMillis();
     Set<UUID> devices = locationEntity.getDevices();
     for(UUID deviceId : devices) {
         location.getDevices().add(deviceId);
     }
     logger.info("Devices in {}ms", System.currentTimeMillis()-c2);

     return location;
}

我的问题是每个位置的设备Map需要大约500毫秒。
将Entity上的devices字段设置为FetchType.EAGER并没有多大帮助，因为这样从DB的初始加载会比较慢。
lctn_location_devices有两个索引：

create index location_devices_location_id_device_id
    on lctn_location_devices (location_id, device_id);

create index lctn_location_devices_location_id_index
    on lctn_location_devices (location_id);

我对JPA的经验不够，我做错了什么吗？