They use “Delegated Proof Of Stake” and claim the ability to do 100.000 transactions a second with Bitshares 2.0. The technology is partly based on LMAX and asks miners to use a great amount of RAM to speed things up. The blocktime could be under a second.
From their website:
To achieve this industry-leading performance, BitShares has borrowed lessons learned from the LMAX Exchange, which is able to process 6 million transactions per second. Among these lessons are the following key points:
Keep everything in memory.
Keep the core business logic in a single thread.
Keep cryptographic operations (hashes and signatures) out of the core business logic.
Divide validation into state-dependent and state-independent checks.
Use an object oriented data model.
By following these simple rules, BitShares is able to process 100,000 transactions per second without any significant effort devoted to optimization. Future optimizations are expected to bring the performance of BitShares to levels similar to LMAX."
More on LMAX in this video.