Delay when double-clicking on files or folders

 Note; free space needs to be 15% (even if 15% is a Terrabyte!), as what is important with HFS volumes are allocation blocks. There are a limited number and if you start using them all up, issues are more likely to pop up.
My advice is:
backup/restore to get full performance back, or
Disk Warrior, the "magician" for HFS directories. Nothing else works in the same way of building directories.

But they are APFS

@sre 
I created an APFS FAQ on our Knowledge base pages, so other users can benefit from this reply:

How much free space is recommended for APFS volumes?

Even with the newer APFS file system, it's still important to leave some free space on your disk. While APFS handles space management better than older file systems, it still needs room to work efficiently.

General Recommendation

• Keep at least 10–20% of the volume free
• Avoid going below 5–10% free space

Why Free Space Matters with APFS

APFS uses a technology called copy-on-write. When files or metadata are updated, the system often writes new data before removing the old data. This temporarily requires additional space.

APFS can also keep snapshots (used by Time Machine and some utilities), which may hold onto disk blocks even after files are deleted.

If a volume becomes too full:

• File operations may slow down
• Snapshot cleanup can become difficult
• Some disk operations may fail until space is freed

For Large Storage Volumes

Even very large disks or RAID volumes should maintain some headroom. A multi-terabyte volume that is 99% full can still run into problems.

A good rule of thumb:

Keeping a reasonable amount of free space helps ensure the filesystem continues to operate smoothly.

I have also found a way to have the issue disappear for a few days. I run Onyx maintenance on the computer. So not sure if that is clearing some cache on that RAID.