I run mine with the actual photos on HDD but the database on SSD. So far everything has been near instantaneous for loading, downloading, uploading, you name it.

Noticeable difference loading the page? Loading photos? Uploading photos?

Photo files are relatively small, so an HDD is absolutely fine.

Usually* there is a database for the file meta-data that will benefit from faster access times of a SSD, the files themselves can be on a HDD.

*not sure how Immich specifically does it.

I use a HDD for my immich instance. I have a feeling it might have made the initial import process quicker (complete google photos dump). However, general usage, I have found zero bottlenecks.

It depends on the load on the disk. My main docker host pretty well has to be on the SSD to not complain about access times, but there are a dozen other services on the same VM. There's some advisory out there that things with constant IO should avoid SSDs to not wear out the read/write too fast, but I haven't seen anything specific on just how much is too much.

Personally I split the difference and run the system on SSD and host the bulk data on a separate NAS with a pile of spinning disks.

Considering the database itself is relatively small, PostgreSQL could end up largely caching it in memory, so even hosting the DB on an HDD might not feel much slower.

Unless somethkng changed in the last few years, SSDs are much much faster.