MacBook Pro 16” 2019

No, it's not the capacity. My drive is 2TB.

 

Currently, it takes 14 seconds for the Apple logo to appear. That's down from when I was doing the dual boot but still not as fast as before I did the dual boot. I'm probably just going to have to live with it since I don't feel like erasing the computer in an attempt to restore the original boot time.

 

14 secs is very good boot time.
Something like Mac OS recovery with filesystem check/repair is worth a shot!

 

Oddly enough, installing Big Sur on a separate volume again fixed the boot time. Now it's instant. It was precisely this that delayed the boot time previously.

 

At least you got it back to good.

 

Unfortunately I couldn't update my MacBook Pro mid 2012 to Big Sur, is there any relevant difference with Catalina?

 

Not in functionality or usage other than using iOS apps on your Mac. This is why I would never buy a macbook. Apple decides when you need to buy a new computer, Not you. I have a 2007 acer 7720 running windows 10 20H2 perfectly fine. Apple is planned obsolescence.