yes, this issue on the Surface sounds to me like a combination of CPU frequency scaling enabled and also passive cooling.
The issue on the new laptop is common with frequency scaling alone. As I said, modern CPUs to my knowledge just can't ramp up their speed fast enough for low latency applications, and the overhead creates a delay that causes buffer overruns.
Re: the CPI, I mean in terms of setting your performance parameters, if you you set Pianoteq's quality settings (sampling rate, polyphony, # of microphones, etc) for real-time use so that they do not create the risk of Pianoteq's processing demand exceeding the minimum indicated on the CPI, you should be safe. This is only relevant for real-time rendering (e.g., while you are playing; not for rendering pre-recorded MIDI files into audio, which can happen outside of real-time and so there is no risk of buffer issues, so you should almost ALWAYS use highest quality settings unless you are going for a specific effect as a mixing engineer or producer...)
If you want to stabilize the processor speed (which is a good thing for real-time or low latency performance because it eliminates the overhead of the CPU takign extra cycles to scale up and down and allows it to devote those cycles to processing user input as well as giving you a more predictable and reliable low latency performance level), you need to lock the frequency at a certain speed. For quiet operation (if fan cooled) and/or battery life, choose a lower frequency. For max performance, choose a higher frequency. Can't have it all...it's a tradeoff.
I am not sure how to accomplish this on Windows 10 but I bet even if there aren't built-in tools to do this sort of performance tuning (e.g., custom power profiles that allow you to tweak the CPU performance settings) there are probably tons of 3rd party (and free) programs or apps that will help you do it. You should be able to do it dynamically in user space (e.g., after bootup and login). Any app or widget that gives you fine control over ideally both CPU min/max frequency and multi-threading (which is ideal to disable in low latency work) should suffice.
I agree with Stephen, laptops very commonly depend on cpu frequency scaling to keep the laptop cool and save battery life because the common user does not notice the milliseconds of latency when browsing/typing/listening/watching that matter to those of us using laptops for real-time or low latency processing work.
They also commonly depend on thermal throttling (which is thermally-reactive downscaling of CPU frequency) to prevent overheating. What this means is: don't expect the CPU to perform at or even near it's "max frequency" setting. For example, I had an i5 quad core ultrabook that was actively cooled via both fan and heatpipe (so, relatively very effective cooling for a small, thin laptop) that had a max frequency of 3.9ghz, but it could only sustain ~2.2ghz for more than a few seconds at a time. Still, that locked the CPI at about 125 for me.
Part of this is another engineering tradeoff: a powerful processor running at full-tilt needs excellent cooling to run sustainably at those higher speeds. Modern thin and light laptops just don't have the space and airflow to dissipate enough heat to do that. So those "high spec" ultrabooks look good on paper, until you need them to use that high performance over an extended period of time (e.g., transcoding or compiling), then you find that they perform significantly more slowly than similarly-spec'd well-cooled desktop computers.
That should give you plenty to google (or duckduckgo if you prefer) to figure out how you need to configure your system
