did you overclock your system? and what is your full specs for you pc.
you can try this for the mean time, use msi afterburner and use the onscreen display thingy option where you can see your temps, clock speed for cpu and gpu. it maybe you are thermal throttling so both your cpu and gpu underclocks to cool it self down.
if thats the case, make sure your cpu cooler is well seated or you can replace the thermal compound (something like thermal grizzly for example) for your cpu and gpu see if theres an improvement there.
you can also but this is very unlikely, do a clean install of your drivers for your gpu. use DDU and just follow the instructions. maybe its some faulty driver issue and is an relatively easy fix.
you can also check if your psu is having and issue again this is another unlikely scenario but it can be a cause. you can try borrowing a good powersupply unit and test it in your system if this heavy performance hit occurs
id point out to trouble shooting your RAM as well but im not knowledgeable about ram issues so i cant give any advice or tips on what to check or look out for.