Problem downgrading from macos sequoia to Monterey on iMac

With the 2018-2020 T2 Intel Macs you must first boot into Recovery Mode to modify the system security settings to allow booting from USB which is disabled by default. You will need to access the Startup Security Utility to allow booting from USB.

About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support

FYI, even when it is disabled, you should still see the correct name for the bootable macOS USB installer on the Option Boot Apple boot picker menu. If you don't, then you may not have created the bootable macOS USB installer correctly. See the Apple article @Matti Haveri linked for instructions.

I completely agree with @Servant of Cats that if you are having issues with Sequoia, then you should try solving those issues since Monterey is much older and you will encounter difficulties with accessing online resources for some apps (especially third party). Plus downgrading the OS is a huge pain, because you cannot utilize any of the items stored in libraries such as Music, Videos, Photos, Mail app, etc. as later versions of macOS likely modified those libraries so that an older OS will no longer be able to understand the new library layouts.

The best way to start trying to solve system issues is by running the third party app EtreCheck and posting the complete report here so we can examine it for possible clues.

How to use the Add Text Feature When Posting an EtreCheck Report - Apple Community

Reset PRAM and try again:

Create a bootable installer for macOS - Apple Support

AFAIR some Macs with 3rd party internal drives had problems upgrading to Monterey. But if the Mac was successfully upgraded to Monterey and later the firmware should have been updated to prevent that in the future.

Before downgrading to Monterey, you might want to check if there is any way to get your Mac running better under Sequoia.

If you downgrade to Monterey,

• You won't be able to run the latest versions of Safari – or even of many third-party Web browsers. (They are now starting to require Ventura or better.)
• You won't be able to run current software from the likes of Microsoft or Adobe. You might not even be able to do a "clean reinstall" of older versions of Microsoft applications that were compatible with Monterey.

If there were changes in the structure of Photos, Music, and TV libraries between Monterey and Sequoia, you might also run into issues trying to use a "new" library under an "old" version of macOS.

Sebastianlaabak wrote:

iMac. It’s a 2019 and it currently has macos sequoia on it, I want to downgrade to Monterey for better performance.

I have Mac mini 2018 and I always kept it at the latest macOS and did not notice any major slowdowns when upgrading from Mojave and eventually to Sequoia.

That said, I do have a small Mojave volume so I can dual-boot from Sequoia to occasionally (very rarely lately) run 32-bit apps like Photoshop CS6 13.0.0, Lightroom 6.14, QuickTime Player 7 Pro, MPEG Streamclip etc.