Your description suggests that you have inadequate RAM for your usage.
The sluggishness matches the description of they system using "swap" memory on disk (instead of RAM). The system uses swap only when enough RAM is unavailable (that's an oversimplification, but still reasonably accurate).
Your screenshot shows TWO web browsers open. Web browsers are notorious RAM hogs. Script-heavy pages like Facebook --and other social media apps-- suck up enormous amounts of RAM. LibreOffice tends to use a lot of RAM, too...especially with complex calculations, formatting, macros, and scripts.
What you can do:
Learn how to use the simple command free to monitor your RAM and Swap use. Any Search engine will tell you how. So will the Search box at the top of this page. So will man free.
Be patient when your system starts to swap and becomes sluggish. DON'T hold down the power button -- that just causes more problems. Instead, patiently close RAM-hungry application, one by one, until swapping ceases.
Better yet, you can anticipate swapping...that's why you learned how to use free: So you know how much RAM you have available, and can adjust your workflow appropriately.
You can probably add more RAM to your system, of course.
Obviously, if you have the LibreOffice scripting skills, you can figure out WHY your spreadsheet consumes so much RAM, and edit the spreadsheet to be more memory-friendly. However, teaching you those skills is well beyond the scope of a simple AskUbuntu question. (The LibreOffice folks might like to teach it)
It's also possible that you have discovered a bug in LibreOffice Calc. However, verifying that bug also requires a certain skill set. Teaching you those skills is well beyond the scope of a simple AskUbuntu question.
