freeing up memory

There are many reasons that contribute to slowing a computer. Here are some.

• You may have insufficient RAM installed. Newer operating systems operate better with at least 16 GB.
• Do you have an antivirus or Mac cleaner app installed? They will definitely slow down your computer and are totally unnecessary.
• If you have a mechanical HD installed it will be slower than a SSD. If this is the case, you can install an external SSD and download an OS to it and use it as your startup disk. This can substantially improve performance.
• Do not keep too many apps open at the same time. That uses up needed RAM.
• Periodically restart your computer. I would suggest doing so at least once a week or so.
• Restart the computer in the Safe Mode. This can take up to 10 minutes while the OS is repairing itself and emptying its caches. (Hold down the Shift key on startup.) Then restart normally.
• We here recommend that you download, install and run EtreCheck Pro. Be sure to allow full disk access. Then post the resultant report here under Additional text. It will provide a comprehensive report of your computer's operation.

I hope some of this helps.

If the User actually meant Empty Space on the Internal Drive.

Manage Storage

It is generally a good computer practice to alway keep at least 15% to 20% of the Total Drive Capacity’s as Empty Space.

Allowing the computer to drop below these guidelines may eventually, cause unintended consequences including what the computer is experiencing now - Slow.

The links below will assist in identifying what is taking up space on the Internal Drive and provide possible ways to remove data that is under the direct control of the User ( Home Folder ) . 

Rebuild the Spotlight index on your Mac

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

OmniDiskSweeper Safe to use

GrandPerspective 

How to delete Time Machine snapshots on your Mac

See used and available storage space on your Mac

Locate backups of your iPhone, iPad, and iPod touch

Notation - If the user is using a cloning software like Carbon Copy Cloner - suggest tweaking the Safety Net Feature in this software. It may be making additional Snap Shots that are not being Cloned to the Eternal Drive. If this should be the case, these Snap Shot could be using additional space on the drive 

The final word from Apple on Managing the " Other/ System Data “ Category

Other / System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

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