Citavi 6.5 only
You inserted highlights, comments, and/or symbols in PDF files with an external PDF editor. In Citavi you then enabled the preview pane option Show all annotations (screenshot).
Citavi 6.5 is considerably slower than previous versions of Citavi 6.
In Citavi 6.5 a third-party component used in the preview pane was updated. This component contained an error, which the publishers have since corrected.
As a workaround until the next Citavi update, disable the Show all annotations feature. After that Citavi should run at its usual speed.