write+sync is a speed limiter in general (even on SSDs), because of waiting for the syncs which are needed for commit durability. On HDD, serial transactions can't go faster than roughly twice the rotation time of the HDD per write, plus seeks, and on network filesystems they can't go faster than four times the network latency (two syncs per commit) plus whatever the backing store needs.

The trick to getting high commit throughput is to allow the durability part of multiple independent transactions to be committed concurrently. On a filesystem-backed DB this means writing multiple transactions per fdatasync. It would be tricky for SQLite to do this because of its architecture as a library and synchronous API, but not impossible.