Looking with PROCMON, saving the file (with dbcs encoding).
Slick writes about 32kb at a time ... but not 32,768 bytes -- seemingly random lengths just under 32,768 bytes each time.
It oughta write 32,768 bytes each time, and keep it aligned.
Also, at the start of the save it appears to truncate the file to 0 bytes - OK, but before writing that much it would help to set the file length to the desired size for best disk space allocation.
V21 writes the same file 8,192 bytes at a time - aligned with DBCS, but offset with utf8 by 3 bytes - but either way it is fast.
This is writing to an SSD.