Why the hardware chokes itself
Throttling is a safety feature, not a failure. Every chip has a thermal limit (many modern CPUs sit around 95-100 C, and GPU hot-spot temperatures often climb toward 100-110 C), and as it approaches that ceiling the controller cuts clocks and voltage to protect the silicon. That behavior is correct; without it parts would wear out far sooner. But for you it means lost performance in a place where there shouldn't be any: the PC isn't broken, it's choosing survival over frames.
How to confirm heat is the cause
The pattern is recognizable: the first few minutes are smooth, then 10-20 minutes in the FPS sags and never recovers until you quit. Run a monitor (HWiNFO, or MSI Afterburner with an on-screen overlay) and watch CPU and GPU temperatures alongside the live clock speed while you play. If temperature pins against the ceiling at the exact moment the clock drops, that's throttling, not a hunch. While you're there, confirm the fans are actually spinning and haven't been silenced by a quiet profile.
What actually fixes it
Start with the free stuff: blow the dust out of heatsinks and fans, sort out case airflow (cool air in at the front/bottom, hot air out at the rear/top), and make sure cables aren't strangling the flow. If the paste is several years old and temps are still high, repasting often shaves a few degrees off the peaks, but do it carefully and don't overdo the amount. You can also make the fan curve more aggressive for extra thermal headroom, and the tradeoff is honest: it gets louder.
Undervolting as a fine adjustment
Once cleaning and airflow are handled and you want more, undervolting lowers the voltage while keeping the clocks. On many GPUs and CPUs this means less heat and noise with little to no performance loss, sometimes even a small gain because the chip stops hitting its limit so often. It isn't a one-click win, though: every chip is different, so the profile has to be dialed in and tested for stability over time, or you'll catch crashes and blue screens at the worst possible moment. If you're not confident, it's fine to stop at cleaning and airflow, or leave the fine-tuning to someone who has done it before.
Verdict
Temperature isn't background noise; it's a direct throttle on your FPS, and no graphics setting will overrule a chip that's sitting at its thermal limit. Dust, airflow, and paste condition matter more than people expect, and fixing them costs nothing. Cooling and monitoring first, fine tweaks like undervolting second, and always verify stability before you trust them.