Steam Deck’s CPU is far more powerful than CPUs of the day when those games were released, the issue somewhere else.

Where do you suspect the problem might be? I know that the memory latency is the primary issue for that engine. It sucked on the CPU of that era, it sucks on the steam deck, but it does not suck for most modern CPU because they have way more L3 cache and better latency outside of it (as the article states)

Fallout 3 has massive issues on any new operating system. There are unofficial patches and mods to fix that but Valve does not auto-install anything unofficial, even if it would make things far better.

I recently played through Death Stranding which is more recent, better graphics and seemingly better optimized than Fallout 3. I'm pretty sure if the CPU can handle Death Stranding (and other more complex games) without problem, it could handle Fallout 3 if the programming was on point.