I have an early 2015 Macbook pro with 8GB RAM and i7 dual core processor. It runs Mojave smoothly, and I haven't upgraded to Catalina nor Big Sur. I read that Catalina won't slow things down compared to Catalina, but what about Big Sur?
If you have enough free disk space, the best thing to do is:
- reduce disk space for Mojave.
- create a Catalina or Big Sur test environment and so you will see for yourself.
The other option is to install the new system on DDE USB-3 and, despite the loss of performance compared to the internal SSD, you can also test without touching in Mojave.
What is important to know is that Catalina or Big Sur no longer support 32-bit applications.
If a Mac is within the included range of supported models for an OS, then it should run without problem.
One of the reasons for Apple excluding a Mac from a new OS is that the 'experience' is not good enough, in terms of speed.
I've run all three OSes on a 2014 MBP, and performance was fine. I didn't notice any discernible difference.