A slow projectile may not penetrate far enough to reach the vitals.
Just looked up BuffLo Bore ammo in 357 mag
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1400 fps and over 800 FPs from 180gr
Hard cast ...
Misses Claus may Not get her New Toaster!
Before smokeless powder was invented, plenty of elephants were killed with 4-bore and 8-bore rifles firing 1200-1900 grain bullets at 1200-1500 fps. And those had to penetrate about three feet of bone, muscle and sinew to get to the animal's brain. The most serious of the modern dangerous game rounds still favor massive bullets at 2200-2400 fps, but always with a non-deforming (usually monolithic) bullet.
Blogger Loose Rounds tested a .455 Webley -- the revolver round the British military used for about a thousand years -- shooting a heavy-jacketed 265-gr conical bullet generating only 559 fps at the muzzle (a paltry 184 fpe) but that penetrated 18.4 inches in ballistic gel. Apart from the engravings from the rifling, the recovered bullet showed little evidence of having being fired, which is typical for a bullet that's meant to bore deep holes.
https://general-cartridge.com/2020/04/07/loose-rounds-webley-455-mk-vi-265gr-fmj-in-clear-ballistics-gel/
I doubt you'll find an experienced archery hunter who'll tell you he's switching to a lighter/faster arrow because it'll penetrate better than a heavier/slower one.