Both of my suppliers told me that recent OS are dramatically heavier and a SSD (SATA or NVMe) is absolutely mandatory.
I haven't slowdowns and issues but compared with Mojave, Catalina with last patch is less reactive when I launch apps and CPU usage from activity monitor is dramatically higher (especially during multitasking).
I think it's basically only an OS issue (just to remember ironically the catastrophic experience on Windows Vista more than a decade ago).
Just to make an example: with the same apps opened with Mojave my inactive CPU is 97%, with Catalina last patch my inactive CPU is 90%; Memory usage at start with Mojave is 1.72 GB, Memory usage at start with Catalina last patch is 2.12 GB.
There are no issues about performance (only less reactivity when I launch apps), but according to my data Mojave is a more performing OS (less input/more output).