The fact that someone knows your browser history and stuff doesn’t mean your phone is hacked. If you use the same account on multiple devices and have sync on, then all this information can be viewed on another device that someone (like your wife :)) has access to, e.g., home PC.
You can’t. The only thing you could do is set the max CPU frequency cap in BIOS, but I doubt it will help if that’s your idle temperature. Clocks are already low in idle.
There is one thing I haven’t mentioned before, so let me write another comment. Did you check temperatures? Maybe stuttering comes from throttling? Modern operating systems lower CPU clocks if the temperature spikes.
Neither CPU, nor GPU would cause stuttering if their utilisation is below 100%. it’s likely the hard drive, that’s causing the bottleneck.
Stutter / lag rather comes from low RAM rather than slow RAM. What’s your RAM usage when you’re experiencing these issues? Aren’t you simple exceeding the limit?
What was the program that you downloaded exactly?
Use the web version of WhatsApp. Open DevTools in Chrome and enable network throttling.