I've a rpi4 that I'm setting up as a media player. Files come from a NAS. In the past (rpi3) I've just used an SD card in the Pi. I'm curious if I'd get better performance using USB. Also curious if I did use USB if I could setup automatic caching to the drive.
I'm using an RPi 4 as a NAS/home server and the difference between SD speeds and USB speeds was pretty noticeable (subjectively); even simple linux commands (ls, cp -r, mv) sped up after switching to a SanDisk Ultra Flair. Except for one USB drive...
Recently I had the misfortune of having a debian update screw up my docker daemon and I ran over to Best Buy to grab any-old-USB drive to use as a new OS drive. As it happens the SD copy over went fine, but as I performed basic "setting the server up" operations like copying over the old data (50 GB of various sizes) and firing up docker-compose scripts, performance deteriorated rapidly to the point where a docker install of a lighttpd container took at least 30 minutes and the system became effectively unusable.
The poor-performing drive was the PNY Elite-X Fit, USB 3.0, 128GB, purchased for ~$22
I've switched back to a SanDisk SD card for the moment which is perfectly adequate for the next few months but I hope to get back to running off a USB soon when I'm not so busy.
For the Pi 3 B+ and previous generations, the difference will be much less pronounced, but still can be measurable.
For the Pi 4, a faster USB drive (e.g. SSD or NVMe in adapter) will be a massive improvement for many things (though some operations like launching a browser won't be vastly improved).
