[I posted this on Sequoia thread, will leave it here too in case it helps anyone]
Just some tips on dual booting MacOS, with the last supported OS on one volume plus an OCLP MacOS on another volume. I'm dual booting my 2012 Macbook Pro and had some headaches of high cpu usage and slowness while switching OSes, after a long time trouble shooting this is what I found out:
Mac OS was never designed to dual boot between versions, it's designed to upgrade the OS only, and therefore when you start up the newer OS it immediately detects an older indexing Spotlight DB and starts to update it to the new version, hence the slowness on start, I was getting the new OS installed feeling every time, seeing a high mds_stores process cpu usage. And when I return to the older OS it detected a unsupported Spotlight DB, discards it and started over the indexing.
There's two solutions for this, you either disable Spotlight on every volume that are accessed from both OSes, by adding the volume on Spotlight Privacy list, inside Spotlight settings. This sets a flag on the volume itself, so both OSes will know not to index that volume. You will lose the ability to search for files using Spotlight for that volume.
The other option is prevent the auto-mount of the volume that you don't want Spotlight touching, by adding the Volume UUID using "sudo vifs", Google this for more precise instructions.
I use a combination of both, by preventing the OS to mount the other OS volume, thus keeping Spotlight, and I have other Volumes for personal files storage with Spotlight disabled, so I can access them on whichever OS is running.
Additionally, for the last supported MacOS to run "vanilla", with all security enabled, you have to uncheck NVRAM WriteFlash on OCLP, rebuild and install OpenCore, and perform the NVRAM reset once, by holding Option-Command-P-R keys when powering on. After that you should only boot to the supported OS by holding the Option key (leave OCLP for newer OS only). This will keep both OS NVRAM settings clompletely separated. You can check the security by running: csrutil status