Which CPU are you using ? How many DRAM is installed ? Do you use a SSD ?

Do you have a dedicated GPU or are you using a cpu with integrated GPU?

What BIOS settings did you perform to disable energy saving completely ?

Are you using the Windows energy profile for highest performance ? Or did you activate the Steinberg energy profile in Cubase settings.

Is CPU core parking disabled? If you use the Steinberg energy profile then parking is disabled otherwise you need to tweak it yourself by editing Windows energy profile's with the tool parkcontrol from bitsum.

What does LatencyMon report if you measure for kernel latency timer (not sitting on my pc from memory). In 3 Min measuring time on an IDLE system in which ranges are the usual recurring timer values? What is the minimum and maximum ? My systems absolute Minimum is 2 Microseconds, usually 10-50 with some rare peaks at around 100.
The tool tells you that your pc is not well suited if these values reach 1000+.
But this is IMHO already a much too high value.
Peaks on an IDLE system (no other app in use) shall be 200 microseconds at maximum otherwise the BIOS /Windows settings are still not good. BTW you need to change the measuring method one time to kernel latency timer to get those values. And remember to NOT start any other DAW/application as we want to catch the basic load and agility of the CPU / system how quickly it can respond to computing tasks.

Before doing any changes on Windows or Cubase perform a backup/diskimage with Macrium Reflect. If something goes wrong then you can restore very quickly to the previous state. Get the payed version, only this version supports "rapid delta restore" algorithm which changes only those disk blocks that need to be changed. Create an usb recovery stick and acticate a restore entry in Windows boot menue.

Bios
disable C-states mostly set to c0/c1
For Reaper use parkcontrol to modify energy profile to not park cores.
Use process lasso pro to bind daw process to around 75% of cores. Increase process and i/o priority. This might help for reaper.
You might also want to use their energy profile bitsum's highest prio.
Install process lasso pro as service if your normal user has no admin rights, then PL can control all processes.
You only need to assign the DAW cores the remaining apps get the rest.
Careful when raising process and i/o prio. Only one level higher, best not too much.

Thanks, I'll try that once I'm at the main PC.

Yes, that's it. Anytime I click outside Reaper audio crackles badly. Moreso if a new process (ie Opera web browser) is opening. Happens lightly on Cubase Pro, Reaper... really bad.

For some reason Microsoft Edge browser causes a lot of crackling on my system that I haven't found a way to eliminate.  The Chrome browser doesn't create nearly as much of a problem.

Sadly Process Lasso Pro needs to be bought if I want total control. Any free alternatives?

I do not know alternatives to process lasso pro.
But they have a lifetime license afair .. So fair enough from price perspective.
Don't they offer a Demo version so that you can try first ?

@widnerm @ramses

Ok, gotta try it tomorrow and see how it goes. I'll report back. Thanks both!

What ecactly did you try with process lasso ?
As you mention very high cpu utilisation over 90%...
You should also be aware of that smaller asio buffersizes lead to higher cpu load and that with a high system/cpu load the demands for lower DPC latency (good optimized drivers and good system settings) are increased.
At a certain point with high cpu load it's not possible to work with small asio buffersize as the cpu needs still be able to process audio in time. With a cpu load of over let's say 75% it will become more and more unlikely to process realtime audio without loss, especially if the recording interface has many audio channels or if the computer has only a fewer amount of cores etc ...

Cubase has an option to enable multi-thread or multi-CPU support, check that. Otherwise there is a clear difference between Cubase CPU display, which only shows the Cubase audio engine working, and the Windows CPU display, which shows the whole system.