I'm having similar issues. I have a large stable FCPX library that is a large feature doc edit with 13 TB of clips external and the library is over 4 Gigs with about 150 events and many projects. I sent a cloned drive to an editor and her's is very very slow and stuck loading.... 4 days of trying everything and it is still rarely get beyond loading after hours and hours.
My system that is what this entire edit had been being worked on from is a macbookpro Mid 2015, 2.5 GHZ i7, 16 GB ram, AMD Radeon R9 2 GB, running macOS Big Sur, 11.6.1 with FCPX 10.6 it is stable and loads fast, like in a minute. then click on something, and it takes another minute or two, loading... then it runs quite well.
I sent a clone drive (G-Tech 20 TB with 5 TB free) with a duplicated library on it to a remote editor. She moved the library to the desktop and has the CACHE going to a separate drive. It opens and all the media is there linked, all path names are identical, but it typically takes hours stuck loading. sometimes finally opening, other times, after hours, a force quit is done, because it just stays stuck "loading"
The machine that is really slow and stalling is running Monterey 12.0.1 and FCPX 10.6. The machine is an iMac late 2015, 4GZ quad core intel core i7, 32 GB ram and an AMD Radeon R9 M395X 4 GB.... So the machine is much faster, more ram, faster graphics card with more memory. We tried the entire list of 12 items from the apple site, like trashing preferences, reseting NVRAM and so on, (https://support.apple.com/en-us/HT203477) reinstalled FCPX. I sent a new Library, with render files deleted, as a disk image, encrypted and compressed then sent by a google drive upload... still the same. Not working. Sent a small library that contains just one event of the big library, and that opens and operates fine. All clips link to the external drive without issue.
Monterey says it wants an Autumn 2015 iMac. Hers is late 2015. Some say Monterey could be the problem. It seems a pain to get her system back to Big Sur. But could that do the trick? Have to wipe the drive clean and start over with Big Sur. The last system was Catalina so there isn't a time machine back up of Big Sur, but seems like this should be tried?
Any ideas? Thanks!