SUMMARY

STEPS TO REPRODUCE
1.  Create a large files with CRLF line endings, i.e. using python:

    open('test', 'w').writelines('a' * 100 + '\r\n'   for _ in range(5000))

2. Comparing the file with any other (or with itself) takes a long time and uses full CPU

OBSERVED RESULT
Freeze happens when lineendings are not unix lineendings.

EXPECTED RESULT
Line ending shouldn't affect load times

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.23.5
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION

From the log messages it seems it's stuck while loading the file, i.e:

    $ kdiff3 test /dev/null 
    org.kde.kdiff3: "Loading A: /tmp/test"

It doesn't get to loading the second file

Also tested with windows (1.9.4 and latest nightly from binary factory)