As sad as it is, because both games are my two favourite games of this generation and amongst my favourite games of all time, both Demon's Souls and Dark Souls had terrible framerate on certain areas.
Trying to walk in the swamp on Valley of Defilement 5-2, while being chased by 3 of those giant goblins and having the framerate almost at single digits is not a fun experience, especially when you're holding millions of souls. And if someone invaded your game, then you were mostly screwed. Teleporting Firestorms are not a fun way to die 
.
Dark Souls also had it's share of very low and inconsistant framerate areas, especially the lava area before Lost Izalith, Blighttown swamp and the Tomb of Giants.
For all the bajillion reasons you could die in those two games, framerate problems were the most annoying ones .
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
