when libreoffice is unstable slow, I would use libreoffice-qt5 instead of libreoffice-gtk3.

Environment

daysleep@t480s-debian ~> lsb_release -a
No LSB modules are available.
Distributor ID:    Debian
Description:    Debian GNU/Linux bullseye/sid
Release:    9.12
Codename:    stretch

Libreoffice's Version
Version: 6.4.1.1Build ID: 1:6.4.1~rc1-2
CPU threads: 8; OS: Linux 5.4; UI render: default; VCL: x11;
Locale: en-US (en_US.UTF-8); UI-Language: en-US
Calc: threaded

Action for Replacement

$ apt install libreoffice-qt5
$ apt remove libreoffice-gtk3