$1800 macbook pro 2019 runs with noticeable poor performance like a core 2 duo pc.

Run the Apple Diagnostics.

Run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper.

The problem may arise that newer hardware that shipped with a specific operating system version may not be supported by an older operating system. If some hardware/system requires Catalina the functionality may not be built in to a previous operating system. Same applies for all computers shipped with a specific operating system.

I don't know if this is an actual case but let's assume that the new hardware (computer shipped with Catalina) requires improved battery health monitoring. Catalina provides that functionality. An earlier operating system version like Mojave may not include that functionality.

If it works for you, great.

Sometimes there are some Apple computers where a computer shipped with a certain OS (lets say it was Mojave). Later Apple release a newer version of macOS Catalina. If Apple did not release a different model/version of the computer, then it is possible to downgrade the OS from Catalina to Mojave if the hardware did not change. Unless you follow the hardware releases very closely you can never be sure if downgrading is possible. Even if you follow the hardware closely it can be difficult to determine which system can be downgraded. That is why it is safest to say you cannot downgrade the OS from that which originally shipped with a Mac.

An older version of macOS may not have a necessary driver for some component. While it may appear to work, the driver may be missing something critical to allow smooth operation. Upgrading an OS is fine since the newer version of macOS will include all the necessary drivers for all supported hardware. Of course once Apple drops support for their older hardware newer versions of macOS are unlikely to work on the older systems Apple no longer supports because certain drivers have been removed from the OS.

This is just the way it works. The same thing happens with every operating system whether it is macOS, Windows, Linux, or any other OS.

Not working here either. Are you using the "Additional Text" tool?

What is ahowing looks like you used the "Link Insertion" tool.

that link is not working for me.

can you post it again?

You can not downgrade an operating system that your computer shipped with.

He downgraded his operating system from Catalina to Mojave on a computer that shipped with Catalina. Can't do that.

FWIW, his report shows for me.

They can't install an older OS than the one it shipped with on any of those computers. You said your computer shipped with Catalina you did not say you upgraded from Mojave to Catalina.