I have a PoC KV store that does > 150M write/s (Rust) using a single core (for 8 bytes of data inserted - it gets bandwidth limited on disk quite quickly even latest NVME PCIE5 disks). The plan is to have it support application RAID0 out of the box so that you could spread the data across multiple disks, but of course that's something you have to setup up-front when you create the DB.
I then would add a SQL engine on top - not sure how much SQL would slow things down but hopefully not much. I haven't found anyone who's interested in anything like that though.
And yes I realize this is several orders of magnitude more performance than any other DB out there.
Similar to @zeroq, I don't really need a K/V store. I need a full relational database that I can also use for analytics and time series, and I want it embedded. And as strict as possible -- PostgreSQL is doing this really well.
I don't mind that there are databases that can be forced into all that and work alright but I admit I am not willing to put the plumbing work if I can avoid it. If I can't avoid it at one point then, well, we'll cross that bridge when we get to it.
DuckDB and ClickHouse(-Local) are amazing candidates but I have never evaluated their normal OLTP performance. For now.
(EDIT: let that not stop you however. Please publish it and announce your thing on HN as well.)
kv doesn't cut my use case. After import I'm running several queries to remove unwanted data. I was shortly contemplaiting filtering while I was importing but (a) I couldn't really handle relationships - hence two distinct steps - remove and remove orphans, and (b) even if I could it's really much cleaner code having a single simple function to import a table, and then having a clean one liners to remove weed. And (c) I need sql queries later on.
I then would add a SQL engine on top - not sure how much SQL would slow things down but hopefully not much. I haven't found anyone who's interested in anything like that though.
And yes I realize this is several orders of magnitude more performance than any other DB out there.