Comment 9 for bug 1826757

From the details provided, this issue seems rooted in package misconfiguration. If you can get your packages clean and conflict free again I suspect the problem will go away. Try again:

sudo apt remove libreoffice libreoffice-core libreoffice-common
sudo apt update && sudo apt install libreoffice

Now, it seems the issue of slow opening ODT files is somewhat known: https://forum.openoffice.org/en/forum/viewtopic.php?f=6&t=86605

Could you try creating, saving, and reopening new ODT file and seeing if the same problem occurs. If it doesn't, you could try copying and pasting all of the broken ODT contents into another to see if the new file loads fine.