It's the common CPU vs GPU misconception. Geforce Now will always use enough processing power to play with the highest graphic quality settings. Those are quite low for HoI4 (recommended GPU is a NVIDIA GeForce GTX570, which is more than enough). This has also no impact on the game's actual speed, which is linked to the CPU. However, Geforce Now likely uses the equivalent CPU processing power that either meets the recommended settings (an i5 750) or supports the GPU that meet the highest graphic settings. I'd bet good money you're getting the equivalent CPU power of the i5 570 (or the AMD equivalent Phenom II X4 955)
Normally, this isn't an issue as most games are more GPU intensive than CPU intensive. Maximising GPU will tend to max out CPU capacity as well. But the service is probably not adapted to grand strategy games where graphics are relatively low maintenance (i.e. the service sees it's giving you a 60fps/1080p stream at the highest graphic settings with no lag using very little CPU power, hence everything is fine on its end and it doesn't need to allocate more resources to your game).
