In my experience all software seems to slow down over time. Firstly, my expectations of "fast" change, but also all the various data structures a system needs to run grow and complicate. In addition we ask more of our software over time and run more of it.

For many, not even Ubuntu is immune. Hence articles like this:

https://www.techradar.com/how-to/how-to-speed-up-ubuntu-1804