dolphin: 500ms delay when closing tab, folder browsing slowed to a crawl since upgrade to 21.10

[Bernd Kreuss]

Folder browsing used to be near instantaneous, especially with nearly empty folders.

Since upgrade from Kubuntu 21.04 to 21.10 it has become next to unusable. Clicking one's way through a folder structure in Dolphin (and also in file dialogs) forces a time penalty of half a second on every click.

Through experimentation I found that it is the **closin**g, not the opening of a folder that has this lag.

I can prove this in three ways:

(1) Changing from Trash bin to ordinary folder is instantaneous, changing from folder to folder has 500ms delay

  * click the trash bin in the places sidebar (500ms delay)
  * click the home folder in the sidebar (instantaneous reaction)
  * click the trash bin in the places sidebar (500ms delay)
  * click the home folder in the sidebar (instantaneous reaction)

(2) split the view and close a folder that is still visible in the second view

  * split the view (instantaneous)
  * navigate away from that folder in the left view (instantaneous because still open in the right)
  * navigate away to the same folder in the right view (500ms delay)
  * repeat with any folder, always same principle

(3) open many tabs and close them

  * middle click a bunch of folders to open new tabs (instantaneous)
  * middle click the tabs to close them (500ms delay for every tab)

There is absolutely no CPU activity, so it seems to wait blocking for some IO. The UI is frozen during these 500ms, mouse hover in dolphin menus or toolbuttons does not react.

I don't know how to debug this further, and I fear I will have to roll back to the snapshot I made before the upgrade because Dolphin feels unbearably sluggish with that lag, so I can't work with it for prolonged time without affecting my psychological stability. So I will have to roll back. (And then I can't be of any help anymore regarding this bug).

Some hints:
* This user has the exact identical problem: https://bugs.kde.org/show_bug.cgi?id=442106

[Bernd Kreuss (prof7bit)]

One more hint, possibly helping to narrow down the cause:

I have these lines at the end of my fstab:

LABEL=Taranis /media/bernd/Taranis vfat rw,nosuid,nodev,noauto,relatime,uid=1000,gid=1000,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,showexec,utf8,sync,errors=remount-ro 0 0
UUID=E098-2BE4 /media/bernd/sdcard vfat rw,nosuid,nodev,noauto,relatime,uid=1000,gid=1000,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,showexec,utf8,sync,errors=remount-ro 0 0

UUID=64f281c4-5799-4608-88e2-cea581f515d8 /mnt/backup btrfs noauto,users,compress-force=zstd:3,noatime,exec 0 0

They are all marked noauto and none of them is currently connected.

If I remove or comment all these 3 lines then the slowness disappears!

If I then uncomment only one of them, for example only the last line (this is an USB drive) and save fstab the slowness immediately returns. If I then connect this drive (without mounting it!) then dolphin is fast again.

So the symptom can be described as such:

* If there is a noauto entry in fstab and the device is not currently connected then dolphin exhibits a delay of 500ms on every click during folder navigation.