We have other constructions in development, NSE and other options. They will be proposed as network upgrades through a FIP (Filecoin Improvement Proposal) after Mainnet launch. We hope to have at least one of these faster and cheaper constructions online within 6-12mo, but the development timeline depends on a lot of different constraints after mainnet launch. Note that new proofs will coexist with SDR, so a new cheaper proof will be optional to miners.