NFS -- very slow reads, much slow than `cp /nfs/path/to/file.txt ~/file.txt`. I generally suspect these have to do with some pathological behavior in the app reading the file (eg: doing a 1-byte read when linearly scanning through the file). diagnose with simple `iotop`, timing the application doing the reads vs cp, and looking at some plethora or random networking tools (eg: tcptop, ...). I've also very crudely looked at `top`/`htop` output to see that an app is not CPU-bound as a first guideline.

ZFS -- slow reads due to pool-level decompression. zfs has it's own utilities, iirc it's something like `zpool iostat` to see raw disk vs filesystem IO.

RAID -- with heterogenous disks in something like RAID 6, you get minimum disk speed. This shows up when doing fio benchmarking (the first thing I do after setting up a new filesystem/mounts). It could be that better sw has ameliorated this since (last checked something like 5ish years ago).