Depends on a few variables. But the pattern is the same, track on a graph both temps and usage of your CPU (might as well do GPU as well) and if you see one of two things:
First frame rate drop and then come back look at the graph, did usage drop and thermal drop and once temp got to a given level performance came back. To fix start with blowing everything out with canned air and ensuring all the fans are working. Fix any that aren't and see if that helps. If it doesn't help and it's the CPU look into redoing the cooler on the CPU.
Second thing you might see is performance is better and then drops and sustains at the lower level once thermals hit a given level. This is most common on mobile devices like a laptop but has been seen on pre-built PCs from some vendors (Dell, CyberPower, most often) in which case upgrading the cooling after the above.
In the end this is unlikely but start looking at the least expensive route first (like this) and then look into upgrading.
As the other responses point out mix-matching RAM speeds isn't a good idea, most mother boards will just run both sets at the lower speed and it's not a problem just not optimal. Some custom boards (Dell most often) aren't smart enough for this and it'll introduce extra stuttering.
In the same app you can track ram usage, if the stuttering is worse when you use more than half than less than half this is most often the cause and replacing RAM so they're both the same speed (preferably from the same company) is the most frequent fix.
Upgrading the GPU is probably next if the above isn't the issue or doesn't get you the performance you want. If you do make sure your PSU can handle the extra draw. Pre-builts (Dell mostly) often put in PSUs that are just powerful enough barely run what's installed and things will start to fail if you exceed it's power output.
Most pre-builts are just fine, don't let elitists convince you otherwise. A few are known for taking shortcuts but most are just fine.
Good luck!!
You're welcome!