Writer - keystrokes delayed

Hi - I experience intermittent issues with delayed keystrokes of the delete key in writer.
The document is less than 100 kB and containes no graphics. Part of the document is imported from Windows/Word.
When editing the document for some time, suddenly LO slows down and keystrokes from the delete key are deffered. It appears that the keystroke buffer is filled quicker than what is showed in the UI dialog: after releasing the delete key LO continues to delete characters (for no obvious reason).

Any help highly appreciated.

Linux 4.15.0-46-generic #49-Ubuntu SMP Wed Feb 6 09:33:07 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

LibreOffice Version:
Build-ID: 1:6.0.7-0ubuntu0.18.04.2
CPU-Threads: 4; BS: Linux 4.15; UI-Render: Standard; VCL: gtk3;
Gebietsschema: de-DE (de_DE.UTF-8); Calc: group