You're assuming the blockchain would work similar to Bitcoin, where every 'trade' (transfer) is a settlement, written into the blockchain. While this is the holy grail, you've rightly pointed out issues with high-frequency trading, synchronization across nodes, etc. The current system does T+3 due to this. A 'blockchain' could conceivably do better - even if it doesn't record every single trade like Bitcoin, it might record transfers at better granularity and better frequency.