OS Catalina slow down laptop

8GB of RAM is the minimum I’d recommend for anyone running Catalina. If you use MS Office and or like to keep lots of tabs open in your browser you will quickly find your computer using virtual memory.

the 866.67GB is storage, not memory and given that amount I suspect you have a 1TB rotational hard drive. That will be a source of slowdown because modern Mac operating systems are tuned for SSDs. Luckily, the 2012 MBP is easily upgraded and SSD prices are low today. What I paid for my 256GB SSD in 2013 will almost buy a 2TB SSD today. That upgrade will make your computer feel new.

There are no pervasive reports of slowdowns caused by Upgrading to Catalina, PROVIDED your computer has at least 6GB of RAM and an SSD Boot drive, and it is operating properly. If you are short of either of those important resources, they CAN be upgraded on the computer you mentioned. Post back if you need help with that or anything else.

Those same computers operate similarly badly if they are trying to run ElCapitan with too little RAM or a rotating magnetic notebook drive. Catalina is only slightly more demanding.

There are no pervasive reports of extreme slowness based ONLY on Catalina.

On your computer, upgrades of about US$150 would make it seem like a whole new computer. Versus US$2500 to replace it.

If you are upgrading from Sierra you'll probably notice a difference but if you are currently running High Sierra or Mojave I don't think you'll notice anything. Catalina doesn't appear to be any more memory hungry than Mojave and I noticed no difference in using my 2015 or 2013 computers when I installed Catalina over Mojave.

"dwb's" reply holds a clue to your answer.

Apple depreceated OpenCL/GL in favour of it's Metal API. Since High Sierra, Apple has changed macOS so animations like the genie effect and window transperency are handled by Metal instead of OpenGL.

This makes older hardware that was released before Metal's existence run like total crap with the newer OS that force hardware to compute Metal which they weren't made to do.

Upgrading to catalina instead of formatting and installing is another way to have a bad time with it. Apple have been making drastic changes with each OS.

The unibody MacBooks are pretty much tanks so if I still had mine I'd certainly replace the drive with an SSD (I'd replace the SATA ribbon at the same time - they tend to wear out over time). The ribbon isn't terribly expensive and if the MBP dies the SSD can be repurposed as an external data drive or TimeMachine drive or some such. At this point though I'd not invest any money that I couldn't recoup through sale or repurposing. In other words, with 8GB of RAM, I'd not add more. That's an expensive upgrade and you'd only get pennies or nickels on the dollar if you tried to resell it later. Same for fixing the optical drive.

So basically, yes it does slow down a computer if it doesn't have modern hardware. Older macOS have a lesser overhead.