Recovery mode? My guess would be that creating bootable media would ensure that no corruption is copied over in case the Recovery image is corrupt? (If it is even possible for the OS image to be corrupt and still run ok but slow, I'm not sure). Would there be any difference (besides convenience), when using the bootable MacOS Installer vs. I am not sure if using Time Machine restore will also possible copy a corrupted state back onto the system, or if this just involves user data, since the guides talk about how Time Machine will also recover the OS. I am not sure which method is best to ensure that I have a clean install when I suspect OS corruption. I am confused because all these guides seem to be for the same result (fresh re-installation of the OS) but totally different methods. Other posts emphasize to boot into regular Recovery mode instead of Internet Recovery mode. Other posts talk about the very simple process of booting into Recovery mode, using the disk utility, and then performing the installation (or Time Machine restore). Some posts first recommend downloading the OS from Apple and installing onto the new SSD when the SSD is connected as an external drive, before installing in the computer.

I plan to use Time Machine to first backup the user files onto an external USB drive. I have a mid 2012 MacBook Pro (A1278) with 10.15 Catalina and it's running extremely slow despite every troubleshooting step, so I want to do a clean install of the OS (in case there is OS corruption) while upgrading the HDD to an SSD drive.

Cleaning macbook pro hard drive