On a given machine any process can run "as fast" as the CPU clock rate (higher = faster = more operations/sec.)

It is true that a single process can do approx. 15-20 MB/sec. (depends on the CPU clock rate & the disk type: SSD, Premium SSD, Nvme)

The best way to overcome this limitation is do to a "heavy I/O" processes in parallel using the queue manager.
On machines with 16/32 cores, you may achieve your "infrastructure limits" (160MB/sec) easily and even more (we managed to go to a 1000MB/sec of Nvme disks)

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Yes, you have also journal files...  they keep all the changes (set, kills, start/end transactions) made to the DB (after actual write to the DBs) and also to be able to roll back transactions.

The write daemon and the WIJ file is more to keep DB physical "integrity" in case of a failure, and its before actual data is being written to the DBs

I see you are using windows. So just look at the windows "task manager" for the "active time" of the disk D:\ If you see that there are times that you hit the 100% "active time" then move the WIJ to a different disk. This will improve performance.