Why is LibreOffice so slow when used in Ubuntu 16.04?

To avoid slow User Interface update, a workaround is to:

1. Remove packages: libreoffice-gnome and libreoffice-gtk and libreoffice-gtk3.
2. Restart (Close and Open) LibreOffice.

After this, LibreOffice User Interface is being updated fast. But, when (main) window is resized, sometimes it becomes messy. So, avoid any unnecessary resizes (resizings), keep (main) window in full size.

It seems to be a GTK port only issue.

Workaround was tested with LibreOffice 5.1.3.2 and Ubuntu 16.04 64 bit. Issue was fixed in LibreOffice 5.1.4.2.

In Ubuntu 16.04, to make LibreOffice Impress 5.1.4.2 fast just remove the libreoffice-gtk package. Don't remove the libreoffice-gnome package, which is necessary for the UI to look good and be properly integrated with gnome (e.g. expected integrated open file dialog). So:

$ sudo apt-get remove libreoffice-gtk

Now copy-pasting slides is fast again, and scrolling the slides as well.

The problem seems to be caused by bugs related to the GTK3 builds of LibreOffice (look here and here). As fare as I found out, the only way to work around is to switch back to the GTK2 version. At first you have to be sure the related packages are installed:

sudo apt install libreoffice-gtk libreoffice-gtk2

Then you can start LibreOffice with GTK2 temporarily with:

SAL_USE_VCLPLUGIN=gtk SAL_SYNCHRONIZE=1 libreoffice

Now it should work again normally. If it does, you can remove the GTK3 package to force LibreOffice to use the GTK2 version permanently:

sudo apt purge libreoffice-gtk3

Shame on Canonical to deliver an untested version of LibreOffice!