Frankly speaking, a 15-second block time makes the network highly susceptible to forking. If a surge of forks occurs, chain reorganization requires extremely high processing speeds; otherwise, if the reorganization takes longer than 15 seconds, the network enters a state where consolidation becomes persistently difficult, making it impossible to reach consensus on a long chain.
A reorganization does not have to finish within one 15-second block interval. What matters is whether nodes can verify and apply blocks faster than they arrive over time.
Parano1d does not replay transactions or rebuild proofs during a reorg. It verifies each {block, HistoryStep} bundle and only rolls back or reapplies the bounded 18-block suffix. This work does not grow with chain height.
A 15-second target still makes propagation and stale-block rates important, but crossing one block interval does not by itself prevent consensus.