Why is a complete, closed libreoffice writer document slow to open?

I have Linux Lite version 3.4, Ubuntu 16.04.4, and my machine is an HP with AMD Athlon II X 4 645 with 16gb ram. My finished and closed Writer (ODT) documents take about 2-3 minutes to open. I’ve never seen this before. I’ve already gone into Tools, Options, Memory, and increased the parameters accordingly, as per on-line advisories.

This sometimes happens on my Windows 7 machine, and I have the impression that there is more than one possible cause. One cause seems to be saving as an odt file with embedded fonts – it then becomes very slow to re-open. Re-saving without the embedded fonts speeds things up.

Thank you so much for your help.
I’ve been creating my documents by writing them myself with regular fonts or by copying and pasting articles of interest that I see on-line. They all seem to be affected. I might have to save what I deem important or new since my last wipe-and-install, and then revert to an early restore point. I do appreciate your insight. Maybe it’s odd characters or embedded scripts or something that’s causing this.

Thanks for your reply. My reply referred to embedding by choosing File > Properties > Font, and marking the box “Embed fonts in the document” which I hope helps when sending editable documents to MS Word users. I then remove the mark and resave the .odt file. For website articles I select the pasted text, use “Clear direct formatting”, then Find & Replace (Ctrl+H) to change all website styles to LO ones. Then I feel more in control. But it would take a real expert to say if I am right.