Would casual be the fastest difficulty for a speedrun, or would it be something different?

Hi!

Yes casual is the fastest difficulty for speedrunning TEW! It saves time in CH1 for example, because you don't have to lure the Sadist away, you can just run right in front of him (watch my Casual mode runs)

Ofcourse enemies are not really aggressive overall, and there are way less dangerous enemies like on Nightmare or Akumu (Ruvik Puppets in CH3 & CH4, Sniper in CH11, only 1 Ruvik Puppet in CH11, etc.)

Also casual mode has the fastest Machine Gun part in CH15 right before the last phase of the final boss fight! Cutcene triggers faster, because the boss takes less damage I guess! Also, if you are really lucky you can 1 shot the final boss during the last phase on casual mode (Happened to me 1-2 times when the game was released, idno if they patched it)