One of the performance optimization technique for insertion according
to Timescaledb blog is to use separate disks for WAL & data.

According to blog [1],
```
Use separate disks for WAL and data
-----------------------------------
While this is a more advanced optimization that isn't always needed, if your disk becomes a bottleneck,
you can further increase throughput by using a separate disk (tablespace) for the database's write-ahead log (WAL) and data.
```

[1] https://www.timescale.com/blog/13-tips-to-improve-postgresql-insert-performance/

Simplified logic of wal_fast can be found here [1] which is as follows:
```
WAL_FAST=$(dirname "$DATA_DIR")/wal_fast
```
Hence, aligning with the same logic to keep things simple. This way, it will not conflict
with separate WAL directory too.

[1] https://github.com/anasanjaria/spilo/blob/master/postgres-appliance/scripts/basebackup.sh