I also had a recent install that was unbearably slow. It turns out that the SD card was corrupted. Raspberry Pi OS comes with a SD card checking utility you can use to check. Replacing it with a new class 10 card made it fast again.
You can boot from USB now. No need for SD.
You should also go for at least Application Performance Class 1.
browsing media is still unusably slow on usb3 hdd as well. once it starts generating previews, youre done. pre-generating doesnt help much
been using it for photo sync only, works fine enough as long as youre not using the web interface
That was my issue. Images would take 3-10 seconds to show in the UI. Once I moved to a ryzen 5 server I was seeing everything load like it was local.