Each of the 64 pieces is fetched from a different host on the network, meaning all of the IOPS are happening in parallel at network speeds. You aren't going to be doing sub-millisecond updates for sure, but you can easily get under 100ms.
The reed-solomon also isn't even the most computationally expensive part, the computationally expensive part is computing and verifying the Merkle roots. All parts of the system though can go >1gbps on standard CPUs.
It's fine from the perspective of a single request but it seems like it reduces the overall throughput of the network. If Sia has, say, 1M hard disks and can do 100M raw IOPS then 10-of-30 gives 10M net IOPS but 64-of-96 gives only 1.5M net IOPS.
The reed-solomon also isn't even the most computationally expensive part, the computationally expensive part is computing and verifying the Merkle roots. All parts of the system though can go >1gbps on standard CPUs.