Nodes can be trusted to be very well-connected, and faults can quickly be fixed by manual intervention, allowing the use of consensus algorithms which offer finality after much shorter block times. Improvements in public blockchain technology, such as Ethereum 1.0's uncle concept and later proof of stake, can bring public blockchains much closer to the "instant confirmation" ideal (eg. offering total finality after 15 seconds, rather than 99.9999% finality after two hours as does Bitcoin), but even still private blockchains will always be faster and the latency difference will never disappear as unfortunately the speed of light does not increase by 2x every two years by Moore's law.
@tetsu – not sure what you mean. My reading of the sidechains paper is that the worst case scenario is that an attacker manages to “reanimate” Bitcoins on the main blockchain that had been sent to the sidechain… but that would be the attacker stealing the coins from the rightful owner on the sidechain. From Bitcoin’s perspective, the coins were always going to be reanimated…. so the risk is entirely borne by the holder(s) on the sidechain. Am I missing something?
The term “sidechains” was first described in the paper “Enabling Blockchain Innovations with Pegged Sidechains”, circa 2014 by Adam Back et al. The paper describes “two-way pegged sidechains”, a mechanism where by proving that you had “locked” some coins that were previously in your posession, you were allowed to move some other coins within a sidechain.
Tú, o el usuario en cuestión de las sidechains, envía los bitcoins a una dirección Bitcoin específica, sabiendo que, una vez mandados, estarán fuera de tu control y fuera del control de cualquier otra persona. Estarán completamente inmovilizados y sólo se podrán desbloquear si alguien puede demostrar que no se están utilizando en ningún otro lugar.
Similarly, a side chain is a separate blockchain that runs in parallel to the main chain. The term is usually used in relation to another currency that’s pegged to the currency of the main chain. For example, staying with the Starcraft motif, say we had an in-game currency called Minerals (oh wait, we do!). We could allow players to peg their Ether (or ETH) to purchase more Minerals in-game. So we reserve some ETH on the main chain, and peg, say 500 Minerals to 1 ETH.
That might sound like a problem, but it isn’t because the box can only be opened infrequently (two or three times a year), and a super-majority of miners must leave a note on the box in advance. This note states exactly where the miners intend to transfer the money. The “correct” note is automatically generated by sidechain software, and is easy to check.
Federated Blockchains operate under the leadership of a group. As opposed to public Blockchains, they don’t allow any person with access to the Internet to participate in the process of verifying transactions. Federated Blockchains are faster (higher scalability) and provide more transaction privacy. Consortium blockchains are mostly used in the banking sector. The consensus process is controlled by a pre-selected set of nodes; for example, one might imagine a consortium of 15 financial institutions, each of which operates a node and of which 10 must sign every block in order for the block to be valid. The right to read the blockchain may be public, or restricted to the participants.

So if you want to create a more secure Sidechain, we would seriously need to have a look at incentivizing miners in other ways. These could include things such as the Sidechain raising outside funding from investors in order to pay the miners. Staggering mining award so miners have an incentive to keep mining as they will be paid later on rather than at the time or the Sidechain could issue its own mining award on top of the already existing transaction fees and essentially just become an Altcoin.
Alpha functions as a sidechain to Bitcoins testnet. The peg mechanism currently works through a centralized protocol adapter, as stated in the sidechains whitepaper. An auditable federation of signers manages Testnet coins transferred to the sidechain. The federation is also relied upon to produce blocks through the signed blocks element. This creates the possibility of exploring the possibilities of the new chain using different security trade-offs.
This is justified by observing that, in our pre-sidechain world, miners always want things to be correct. In theory, the incentives of miners and investors are very strongly aligned: both are compensated most when the exchange rate is highest. And, in practice, we do not see large reorganizations (where miners can “steal”, by first depositing BTC to major exchanges, then selling that BTC for fiat (which they withdraw), and finally rewriting the last 3 or 4 days of chain history, to un-confirm the original deposits). These reorgs would devastate the exchange rate, as they would cast doubt on the entire Bitcoin experiment. The thesis of Drivechain is that sidechain-theft would also devastate the exchange rate, as it would cast doubt on the entire sidechain experiment (which would itself cast doubt on the Bitcoin experiment, given the anti-competitive power of sidechains).
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Sidechains have been a concept for a relatively long time in the cryptocurrency space. The idea took flight in 2014 when several eminent figures in cryptography and early digital currency innovations published an academic paper introducing Pegged Sidechains. Several of the authors are central figures at Blockstream, who is at the forefront of innovation in sidechains and other Bitcoin developments.

Cabe destacar el papel de la gente de Blockstream, una de las compañías centradas en la búsqueda de este objetivo (con un extremeño en sus filas, Jorge Timón). Blockstream está trabajando actualmente en el desarrollo de un protocolo que permita crear sidechains. Son los responsables de uno de los papers más conocidos sobre el tema, publicado en Octubre del 2014:

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As RSK plans to host all types of clients and smart contracts: financial industry players, educational institutions, large importing companies, government and individuals, which means they are full on attack mode on Ethereum’s business model. There are endless opportunities within a market with unlimited potential and we could now see a first real competitor for Ethereum, that has a big hashrate, secure network, safer environment for developers, much higher throughput and solved scalability issues.
Cohen said Walmart also has a patent on drone delivery systems that facilitate orders in a cleaner way, track package contents, environmental conditions and location. Walmart supplier Coca-Cola is starting a pilot to use blockchain to identify inhumane labor conditions in its sugar supply chains. Coca-Cola plans to create a secure decentralized registry for workers and their contracts to help securely record their workers’ identities while providing a trail in case employers abuse their power.
Private blockchains, or as I like to call them, shared databases, have a place in improving efficiency for financial institution for back-office settlement processes. They should not be seen as controversial, or part of some dialectic struggle between punks and police. To the extent that the identifying shroud of AML/KYC can be placed into public blockchain metadata (possible in Omni Layer transactions over the Bitcoin blockchain) there may even be interoperability between these two sides of the train tracks. Right now, due to state-granted monopolies to issue credit, most of the world's liquidity is still in banks. However, we believe that in the long-term, public blockchains, especially those based on work, will come to take a more significant part in the ‘System D’ informal economy, which is where most of the global economic growth will originate.” 
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