How can I force a crash report in OSX?
Activity Monitor
If you just want a live view of system activity, keep Activity Monitor open and change its Dock icon to CPU to see if cores are busy. You may of course open it and see what is hogging the memory/ disk/ CPU.
You can either enable memory column in CPU tab, or CPU column in memory tab to keep a tab :) on processes.

Last time I had such a slowdown, it was spotlight processes consuming massive memory. I killed it.
Instruments.app
It is bundled with Xcode app which you say you have. It has an instrument called Activity Monitor which will store the information of all processes, network, disk speed for as long as it is run. Activity Monitor app will not store anything.

https://web.archive.org/web/20200620073428/https://help.apple.com/instruments/mac/current/#/devc63117d3
https://en.wikipedia.org/wiki/Instruments_(software)#References

Safe Mode or new user account
Not a logging method, but can help isolate issues with kernel extensions, buggy apps or anything else.

https://support.apple.com/en-us/HT201262#issue