FCPX painfully slow and jumpy playback

Render (print, bounce) the problematic section by selecting the clip or clips and pressing Control-R.

Manage render files in Final Cut Pro for Mac - Apple Support (English)

Also, if it hasn't already been mentioned, change playback from Better Quality to Better Performance.

Control playback quality in Final Cut Pro for Mac - Apple Support (English)

In addition to what Ian said: if playback is not smooth you can switching from Better Quality to Better Performance in the little View menu at the top right of the viewer.

Please give details about your mac RAM, drive space (as Ian correctly pointed out, don’t want to work on a full drive).

Also, if working with an external drive, that may the bottleneck if it is not an SSD.

There is also a chance that those PFS templates may cause some difficulty.

For a more complete portrait of your system: Please run Etrecheck and post its full report here.

Use the “additional text” button and paste the report into the text box.

there could well be other issues (I can almost say the name of a certain web browser…)

I'd start by following the instructions at https://chromeisbad.com to completely remove all the files that Google scatters across your drive. Once that's done, reinstall a fresh copy of FCP from the AppStore.

I also see some CoreAudio crashes. You might run a test without them by temporarily disabling AU plugins before launching FCP. Audio Units are in /Library/Audio/Plug-Ins/Components/  Rename to something like Components.ignore to have FCP bypass loading.

Finally, I see Topaz is installed. If it's applied to clips it can be extremely processor intensive.

Is your SSD nearly full? You need at least 20% free for efficient editing.

Try deleting the preferences by holding down the Option and Command keys as you launch FCP until a window appears. Click the "Delete" button.

Another possibility is a corrupt file in the project.

I notice that you mention the scene removal mask. This uses a lot of neural power so I would suggest that when you have used it to isolate your subject you add a green generator to the background, export the video and reimport it to use in your project as a chromakey clip.

This will use very little processing power.

Duplicate the project, then in this duplicated project, select All and remove all effects (Option-Command-X). Does it playback without issues?

Do you know what is taking up the space in your Library?

Navigate to it and right-click.

Select Show package contents.

Then go to the View menu in the menubar and choose Show View Options>Calculate all sizes.

With that large library selected, did you do

File->Delete Generated Library Files?

The menu changes depending on what is selected. It could clip, project, event or library.

And also: did you quit FCP after doing the above? The size change won’t appear until you close the library or quit the application.

If you have Chrome installed it's worth reading and acting on this article.

https://chromeisbad.com

Delete your generated library files from the File menu. Use the consolidate function to move the media to a location outside the library bundle.

After following Tom's always excellent advice, make sure to disable background rendering in FCP Settings. This can generate enormous files that are typically not necessary.

Luis has tactfully not mentioned that CleanMyMac and Chrome come high on the list of "usual suspects" when FCP develops performance issues.

In Settings check the background in playback. Set it to checkerboard to see if it’s transparent.

Please show your library properties with the location settings. There is no way a library can be that large if it has no generated files and no media in it, unless as Been said there are ML analysis files or huge stabilization files in cache.