Any reference about the 'no discard' advice ? I used it for a while on an Intel 520 80GB SSD, it was fine at first but when I reached low free space (~1GB) the SSD became sluggish, which isn't unexpected, but even after freeing 4GB it was still slow.
Manually trimming reported '0 Bytes trimmed'. Only after remounting the disk without `discard` and then `fstrim --all` that the SSD got decent performances back.
I'd guess your SSD was doing a synchronous discard which would be slow. I know anything less than SATA 3.1 does it synchronously, and maybe there are other situations too. E.g., maybe async trimming requires a decent amount of free space?