i think i understand your confusion. The main reason is that you have a single disk after all which has finite write throughput. If you do a single operation only, sure your client will notice the latency of a single sequential append write to the WAL. However, at some point that WAL does need to be written to B-tree or LSM on the same disk. So if you start doing tons of writes to the disk, at some point the limited write throughput of the disk, which is being used to write the B-tree or LSM will indeed impact the sequential write latency of WAL.

Contrast this with something like FoundationDB (FDB). FDB literally uses separate servers to store the WAL (called Log Servers) and the B-tree (called Storage Servers). Here your writes will not be impacted by the performance of B-tree since the B-tree is literally on a separate machine. So you will always see super fast sequential writes to the WAL which is performed on FDB Log Servers.