FCPX slow, every basic function is extremely slow :(

The problem with the clip browser (every time FCP is launched) is indeed a bug that I and others have known about for many years.
It has occurred on all my Macs using various operating systems even though I clean install the OS every year or so.
Many people will not notice it because it only appears once a clip reaches 2 minutes. Most of my clips are under 2 minutes long so it is rare for me to experience the effect . . . especially as I usually leave FCP open when I put the computer to sleep at the end of the day.

From the comments you have made during this extremely long and convoluted thread, it would appear that your project is both massive and unwieldy and contains many problems that have been compounded over the years.
Your computer, not just FCP, is probably clogged with loads of crud that has amassed over time.
I suspect that anything we could suggest would be akin to slapping some paint on a car that is a write-off.
Under those circumstances it would probably be simpler to completely erase your computer and install the macOS afresh.
Keep all the original media of course but start the project again.
As you will have all the assets plus a knowledge of the shape of the film, you could probably edit it very quickly . . . so what may have taken years to amass will likely only take a day or so to redo.

@inbro

Cheers.
> Your computer, not just FCP, is probably clogged with loads of crud

I don't recall ever documenting that my computer was in any other way behaving buggily.  I only clean-installed my current OS - *clean* *installed* - within the last few months.  I zapped everything.  There is no "years of crud".  There is "months of use".  The *project* and its containing *library* are the things which have suffered strain and corruption over years.  

>I suspect that anything we could suggest would be akin to slapping some paint on a car that is a write-off.
Under those circumstances it would probably be simpler to completely erase your computer and install the macOS afresh.

See above.

I've also clearly documented that days ago I tested this project on a brand new M1. In this context, I'm not sure I'm understanding the rationale for erasing my HD and starting again.  

Even FCPX most of the time is working fine (taking aside the gripes I have).  It's only this project, and again, it's been well covered here what the problem is.  I think @davem and @joema have covered it all comprehensively.  I did indicate in one of my last posts in this thread that i'll use the advice I've been given, and will report back any developments. 

Appreciate the thoughts.  Cheers

Apologies for mis-reading your problem.
If your computer is OK it sounds as though it is simply the project that is filled with problems so as I said before, I would be tempted to delete it (saving all the original media) and start afresh.
It is surprising how quickly you can re-edit a project that has taken ages to amass.
This is because much of the time is often taken up by deciding what to do and experimenting with different ideas.
When you start afresh, you have already gone through the "thinking" process and can speed through the edit.

Slaughter wrote:

... I de-installed Chrome completely and it has made no difference, but I note that *any* beachballing - in *any* project - is often "unstuck" by opening youtube in Brave browser and toggling play/stop. Is this a known phenomenon? It's completely bizarre to me...

If you EVER had Chrome on your machine and did not de-install it via the procedure mentioned in the below article, it might still be causing problems. Supposedly Chrome or other related Chrome components can saturate the MacOS Video Toolbox framework, causing poor FCP performance. 

In theory Brave should be OK, however it's conceivable Brave (which is based on Chromium), might somehow be triggering residual installed elements of Chrome.  Provided Chrome is fully uninstalled using this procedure, Brave should be OK. If you haven't manually uninstalled Chrome using the below procedure, try that.

If others know differently, please elaborate.

Chrome deinstallation procedure:  chromeisbad.com/#delete
Main Chrome article:  chromeisbad.com

Many thanks!
There were 3 components which AppZapper missed. I will report back any changes this might bring about.

This may be the answer I have had a nightmare with the damn beachball recently
it would appear after every simple tiny move of the mouse even and then stay there for thirty seconds, more sometimes longer, then hang until I had to restart Final Cut. it was SO maddening.

The fix was turn off Background Render in preferences and lo and behold no more beachball. and back to snappy editing

Thanks Ian - yes, er, we've kinda already covered this but given the massive amount of data that is in this thread I understand you probably haven't had a solid read through.  Cheers though!