Raspberry Pi 4 sound quality

[Miss Sissy Princess]

Disk speed test using G-Tech Slim 500MB external 5400. USB3. Formatted as ext4. Connected to USB3 on Pi-4.
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So what you're really measuring is the performance of the Pi-4 as a NAS for your Mac, both on WiFi and hardwired, not its USB 3 disk interface speed, right?

If the drive with the music is directly connected to the Pi-4 by USB3, how the Pi-4 performs as a NAS is immaterial to the sound quality (and this is the Sound quality sub-forum).  What matters there is the local hard drive performance of the Pi-4.

If you do want to evaluate its performance potential in a NAS role, then adding in bandwidth-limited devices like the Airport Extreme just muddies the waters.  Directly connect the Pi-4 to your Mac, first via Ethernet, then via 5GHz WiFi, and finally by 2.4GHz WiFi.  Run the benchmarks in each configuration.  Don't throw the Airport Extreme (and all of your other network traffic) into the mix.

Why did you want the Pi-4 volume to be formatted as HFS+?