Yeah I took a 2012 iMac , loaded Ubuntu and it runs latest Firefox and chrome like lightening. Take same 2012 iMac and run latest macOS it is allowed to run and it runs super super slow.

This is clearly by design. They like to advertise long support windows but they cripple older devices through software and OS features causing older devices to run much much slower.

I have seen this for years from Apple and it would be hard to convince me that this isn’t done by design.