As soon as what you have in your machine doesn’t literally match the stated system requirements, you’re on your own. It’s up to the user to research and understand which CPU or GPU is ”better” or ”worse” than the required one. These things are nontrivial when comparing between generations and across tiers, not to mention across different vendors.
A knowledgeable user might be able to predict their performance reasonably well, based on publicly available benchmark databases, but you still can’t really get a good estimate FPS unless you find someone with exactly your hardware setup who benchmarked the game (and is willing to share).
Most minimum/recommended game specs reference mainstream gaming CPU/GPUs, and most gamers know the strength of their own hardware relative to mainstream components.
If you're a very casual/young/inexperienced gamer then sure, you might have trouble comparing your own system with the min specs.
A knowledgeable user might be able to predict their performance reasonably well, based on publicly available benchmark databases, but you still can’t really get a good estimate FPS unless you find someone with exactly your hardware setup who benchmarked the game (and is willing to share).