New iMac unbelievably slow
You are on the correct track to make specific benchmarks and time them.
So if you have “open system preferences” you can repeat it. If it takes 30 seconds to open 5 times in a row, you could run some tests to see if it’s network or disk or CPU that’s constraining it.
If it speeds up the second and third times, then it’s a different issue.
Can you reproduce this in a new user account on the Mac with no startup items?
Either way, using the first two Activity Monitor functions to end high CPU use processes and see overall energy use should out any apps causing a slowdown. Also, do you need more ram?

https://support.apple.com/guide/activity-monitor/welcome/mac