Hi Raoul, I had not found a solution until today. I upgraded to Cubase Pro 11. On my Intel i9 7900X based system, it eventually performs efficiently, using ASIO-Guard, Level: Low (17ms additional latency). In Cubase Pro up to version 10.5, even with high ASIO-Guard setting, spikes were a major problem with every project, irrespective of number of channels. Looks like Steinberg has resolved the problem in the latest version, but I’m also thinking it’s unacceptable to be asking people to upgrade in order to make the software usable…
Having said that, I am not suggesting you should upgrade to the latest version, as I can’t be sure that your system is affected in the same way mine was, but if you normally upgrade, I highly recommend that you upgrade sooner rather than later. Version 11 addresses several other issues, including support for Hi-DPI displays in Windows.
I hope you find the above useful.