Why does Dolphin take forever to load on Kubuntu 20.04 all of a sudden?

I am using Ubuntu 20.04 with kde plasma. All of a sudden, my Dolphin file browser takes forever to open. Earlier it opened instantly, but now it is taking almost 30 seconds to start.

This seems to have started after an update; when I use kernel 5.11.0-38-generic there is no issue: the problem occurs when I use kernel 5.11.0-41-generic. In addition when on 5.41 the whole system seems to run slower.

When I run dolphin from the command line, there is no error output, and it is as slow as before to open.

I am using an Intel 4690k CPU with 16GB ram and SSD. I checked my drive health and it is fine.

Dolphin was giving me problems on and off, and I decided to look at 'System Monitor' to see if I can see anything that looks off. Sure enough I saw a bunch of Dolphin opened in the 'Processes', even though I was only running one(It could have been in the 'Applications' part, i don't remember). So I stopped them, and Dolphin came back to normal...

The kernel versions you are quoting are very strange. Please add in the question the output of

$ uname -a
$ lsb_release -a

in both cases. Now, a few things to try:

1. pkill gvfsd-trash is proposed here as a solution. If that works, we may try further understanding. Further diagnostics shown here.
2. systemctl mask upower.service is proposed here as a solution. If that works, we may try further understanding.
3. Check with top and strace -c dolphin (and exiting dolphin) if that can give a hint of the cause for the lag. (related)
4. What do you get if you run dolphin from the command line? Please post in the question. (related)
5. baloo, loaded with dolphin, is often a problem, but it shouldn't be different among kernels. Could you confirm?

Related:

1. https://forum.kde.org/viewtopic.php?f=224&t=160306

There's a lot of things this can be but you need to rule out the possibility that your system is experiencing a hardware issue. It's the type of thing that if you don't get on top of it right away you can experience data loss. Check /var/log/syslog for disk/SMART errors ASAP.

Here's how to check the SMART status of your drives... Install some SMART-checking packages:

sudo apt install libatasmart-bin smartmontools

Now run this command for every disk you've got:

sudo skdump /dev/sda # Replace sda with your disk(s)

This is what you're looking for:

SMART Disk Health Good: yes

If you want more details about the status of your drive you can use smartctl:

sudo smartctl --all /dev/sda

Though honestly, unless you really know what you're doing that output isn't usually as helpful as skdump.

There's a few other more mundane things that can really slow Dolphin down:

• A directory with a ton of files in it. So if restores the last open tabs when you open it and one of those directories has a ton of files... There ya go. Change the tabs to stuff that's quicker to open and close Dolphin so that next time it'll open fast. Alternatively, just configure it to "Show on startup:" your home directory instead of restoring the last session's tabs.
• A network-based KIOslave/remote directory that's currently down or just has a lot of files in it (or is just generally slow like some webdav thing). Same sort of problem as the last bullet.
• It's opening a directory with lots of files that can have thumbnails/previews that aren't getting cached (or can't be cached or the cache is being regularly deleted). Normally Dolphin generates previews in the background while you're looking at any given directory but if it thinks there's cached previews/thumbnails for a large directory of files and all those files are now missing that can really slow it down.

Another thing to check: Instead of opening Dolphin from the menu/launcher open up a terminal and just run dolphin. That way you'll get to see any warnings/errors as they occur while you're using it. You can ignore most QT-related warnings/errors like, "BadWindow". Look for words like, "timed out" or "timeout" as that can be a smoking gun.