Dolphin is not caching the previews of files on LUKS encrypted volumes. This means they are regenerated upon every navigation which is incredibly slow.
From what I gather this is to stop “leakage” onto an unencrypted volume (/~/.cache/.thumbnails)
However, I would like these thumbnails cached. Is there a way to achieve this?
KDE Plasma 5.27.11
KDE Frameworks 5.115.0
Qt 5.15.12
Yeah stop using encryption or storing your oh so important files that you want cached to appear for on an encrypted volume. You cannot not have it both ways. It is like being a little bit pregnant it does not happen like that, it is an either or situation. You have security of encryption or you do not, pick your poison and live with it.
An unpleasant and needlessly aggressive reply 
Surely it would be nice to have the option? Or the cache could be relocated on to the encrypted volume?
I would much appreciate any helpful advice.
Hi - not sure myself how to approach the issue, but just for reference in the discussion - there are two previous and one current KDE Bugtracking System reports that stick out as having potentially relevant context here about when thumbnailing is allowed and where it should be performed:
Yes you are so hard done by with me pointing out the folly of your question.
Hi Johnandmegh
Thanks for those links. The solution seems to be that if your /home is encrypted then what I proposed is quite feasible. In this case I moved my /home to a partition encrypted with LUKS2 and bingo the thumbnails are cached from other encrypted volumes. Much appreciate your helpful input.
JohnBrown - put that in your pipe and smoke it.