aving researched and tried many things I have eventually made a small change and believe my system now works fine. I have a bottom of the range 2019 Dell G3 3590 with i7 9750H CPU and Cubase Pro.
Very briefly, LatencyMon showed ACPI.sys to be the main culprit of spikes occurring on CPU 0. There are lesser issues too and lots of advice out there about Bios settings, Windows configurations, power settings and how the DAW is used.
As far as I can see, the many tweaks make little difference so I've left things standard except:
1) I launch Cubase so it uses all CPUs (0 to 11) except 0 and 1 using the shortcut "C:\Windows\System32\cmd.exe /c start "" /affinity FFC "C:\Program Files\Steinberg\Cubase 10\Cubase10.exe". Note that /affinity is set to "FFC" (hex) as I want to use CPUs 2 to 11. This alone appears to have killed ACPI.sys spikes and results in steady low CPU usage. Affinity can be set for your specific CPU on the Details tab in Task Manager every time you load Cubase or automatically using software, however, this shortcut is free. You need to set Affinity (the "FFC" part) according to the number of (virtual) CPUs you have (see below).
2) For a little extra comfort, I sometimes disable the Wifi adapter. On my desktop I have a shortcut to Control Panel - Network Connections to make this easier.
3) For a little more comfort, in Cubase I have recorded tracks routed to one (or more) multi-channel VSTi (Halion Sonic SE) rack instruments, and my live keyboard track is routed to a separate rack instrument to reduce the CPU load on live playing. Secondly, I also use the Steinberg Audio Power Scheme under Studio Setup - VST Audio System. Thirdly, I still have the option to "freeze" the recorded tracks VSTi but with CPU use averaging 20% it seems unnecessary.
4) C-States, Hyper-threading and other Bios settings and all other tweaks I read about appear unnecessary. The main issue for me appears to be the occasional spikes on CPU0 and sometimes CPU1 so I force Cubase to use other CPUs.
To calculate affinity:
1) I worked it out in decimal (result was 4092) to use cores 2-6 (CPUs 2-11) using store.tomandmiu.com/cal-cam.php
2) I converted the decimal result of 4092 to hex (result was FFC) using www.rapidtables.com/convert/number/decimal-to-hex.html
My CPU now appears to run steadily at about 10% to 20% or less when playing several MIDI tracks and live keyboards with an ASIO setting of 256 (it will run fine with ASIO much lower too).
Hope you have some luck with this.