It's a very beefy server with 4 NVMe and 20 HDD bays + a 60-drive external enclosure, 2 enterprise grade HBA cards set to multipath round-robin mode, even with 80 drives it's nowhere near the data path saturation point.
The link is a 10G 9K MTU connection, the server is only accessed via that local link.
Essentially, the drives being HDD are the only real bottleneck (besides the obvious single-node scenario).
At the moment, all writes are buffered into the NVMes via OpenCAS write-through cache, so the writes are very snappy and are pretty much ingested at the rate I can throw data at it. But the read/delete operations require at least a metadata read, and due to the very high number of small (most even empty) objects they take a lot more time than I would like.
I'm willing to sacrifice the write-through cache benefits (the write performance is actually an overkill for my use case), in order to make it a little more balanced for better List/Read/DeleteObject operations performance.
On paper, most "real" writes will be sequential data, so writing that directly to the HDDs should be fine, while metadata write operations will be handled exclusively by the flash storage, thus also taking care of the empty/small objects problem.