I made a guess when I selected frameworks-kio as the offending product. Please correct if wrong.

SUMMARY
kde-frameworks-5.82.0 allows changing folders within dolphin and kate, without any freeze.
Using kde-frameworks-5.85.0 and kde-frameworks-5.87.0 results in a 20 sec freeze when changing folders 
within dolphin and kate.

STEPS TO REPRODUCE
1. Install kde-frameworks to 5.85.0 or 5.8.7.0
2. Start dolphin

OBSERVED RESULT
1. switch selection between two folder containing a large number of files/folders
2. observe the freeze ( 20 secs on my box ) before dolphin is responsive again.

EXPECTED RESULT
1. switch selection between two folder containing a large number of files/folders
2. no freeze occurs.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma:  Linux-5.14.15-gentoo-x86_64-AMD_Ryzen_9_3900X_12-Core_Processor-with-glibc2.33
KDE Plasma Version: 5.22.5
KDE Frameworks Version:  5.85.0 or 5.8.7.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
First reported with at https://bugs.gentoo.org/show_bug.cgi?id=821103

The folders tested were the Documents and Downloads folder in the $HOME directory, formatted with ext4.

During the 20 second freeze, high IO ( > 50% ) can be observed on many hard drives via atop. Interestingly typing blkid in konsole takes about 14 secs to return.

I upgrade kde to the latest stable release on 13th Sept 2021. Everything seemed to work OK at first but
I later found that when I use dolphin or kate the application freezes for 20 secs when changing to a different folder.
I downgraded the whole of kde to the release in use prior to the 13th Sept 2021 and the issue disappeared, confirming that 
the problem was in the latest stable release of kde.
After some thought I upgraded all but kde-frameworks to the latest stable release of kde leaving 
kde-frameworks at 5.82.0. There was no 20 sec freeze.
Wondering if the unstable kde-frameworks 5.87.0 might also be OK I upgraded to that but the freeze returned.

I also noted that when starting dolphin from the command line with kde-frameworks-5.85.0 a large number of errors were reported on the console. However kde-frameworks-5.87.0 did not manifest this so is not relevant to this issue.