Docker is NOT an “improvement of LXC’s capabilities”, because it does not provide same service, it’s application isolation, while LXC is subOS isolation.
– LXC=container for normal multiprocess OS, sysadmin tool, keyword=isolation;
– docker=application container for single process, programmer tool, keywords=staging,delivery.
If you need performance and “cheaper VM” – you should go with LXC. From technological standpoint, LXC is more correct implementation compared to docker.