Yeah its an achilles heel with PC gaming at the moment, the observations I have made do vary from game to game.
UE4 games are prone to having stutter issues when dynamically loading assets be it be shaders or textures in exploration type games where things have to be streamed in.
Some games it might be due to them been restricted to one or two threads only and then becoming CPU bound, often these games seem to have kernel overheads as well, meaning if you tell task manager to show kernel cpu time, you will see a large amount been allocated to the kernel on the loaded cores. My assumption is it might be context switches but hard to diagnose on windows.
i/o can be bottlenecking, this will apply likely to the UE4 reason as well, sometimes it might be the storage device is too slow to stream in, but it also could be its too fast which can cause temporary cpu saturation, e.g. in ff7 remake I was able to mitigate the stutters by playing the game of a spindle instead of a nvme ssd. The latter should be solved by directstorage.
Background software, particularly security software as well as exploit protection can be a trigger. This one will be controversial, because turning off things like control flow guard, might be considered snake oil, most people who report improvements dont show proof, and it makes you more vulnerable to malware.
Interrupts get loaded up on the first core, which is why some people suggest adjusting cpu affinity to prevent that core been used in games, I personally do find this to be beneficial, but also can be problematic, e.g. in Zestiria I killed all the micro stutters by disabling core 0, but then instead every so often I get a temporary freeze, the game will stop for about a second and then carry on, I eventually discovered when it does it somehow forces itself onto core 0 temporarily then unfreezes when it changes to an allowed core again.
Hardware USB devices can cause stuttering in games as well.
Other less complicated causes of stutter.
Power management, modern CPUs and GPUs by default will downclock to save heat and watts. In particular the current Nvidia drivers are actually quite bad at this and it might be more noticable on high end GPUs. Basically before Ampere there was 3 power modes in Nvidia drivers. Optimal, Adaptive and Prefer full performance. Adaptive was the default, and had fairly aggressive ramp up of clock speed to minimise the chances of the clock not been high enough to prevent frame drops. Optimal was similar accept kind of like a conservative adaptive option when it favoured lower clocks, in this mode in my experience it often cause slowdowns when transitioning from low demand to high demand areas of which the GPU didnt ramp up its clocks fast enough, finally prefer max performance was also adaptive, behaved like the adaptive profile but forced the card to always stay in 3d clocks so prevented the clocks going down very far. But on launch of Ampere cards some AIB cards were unstable, Nvidia fixed this in the drivers. The changes they made was they removed the previous default Adaptive completely from the drivers, Optimal is now the new default and renamed to "Normal", prefer max performance has been tuned so it now forces boost clocks instead of base 3d clocks. These changes suggest the cause of the instability was rapid large voltage changes were triggering issues, but anyway the result of these changes is that the default "Normal" profile is more prone to clock speed lag causing stutters in games e.g. you in a menu that has 2% load on GPU, it downclocks, you exit menu and suddenly game has to fully render 3d objects again, you get temporary circa 100% load on GPU before it ramps the clock speed back up.
A very easy way to consider if power saving is the case is set Nvidia to "prefer max performance" for the game, and also setting ultra performance in windows for the CPU, even maybe setting min CPU speed to 99%. However if you have Ryzen, I think cpu clock lag is unlikely to be the cause as ryzens dont clock as low as intel chips and are also much more aggressive at ramping up.
This is all assuming its not vsync related which is a common cause. In addition if you playing games with uncapped framerates your gpu may not ever be entering low power state.