So, playing in browser. I know it's not optimal but the problem I'm seeing is that the lag isn't "the first time shaders are loaded", there's lag every time the mist effect is used. I assume what you're doing is loading it in every time, rather than preloading the scene and then instancing it in every time the mist needs to be used. I'm not used to using Godot for browser games so I'm just making assumptions here, I could be wrong. This goes for many other effects too, if you're loading into a dungeon that has ratkin you should be preloading any assets they use.
Buffs/debuffs in games are always going to be an uphill battle of making them clear and readable. I noticed more than one buff that has the same(?) effect of 0.25x damage. Saying "x25% damage" is a bit weird, the icons are rather small, there's no effects to go with the buff. It's good the buffs have significant effects and I can understand and read the buffs but if a dev has time they can always add more effects and visibility to stuff like that.
I didn't play a lot since I'm pretty busy but I might do some more, sometime. I think the use of voices is good, using darkest dungeon traits/vices after missions has potential, lots of potential for lewd content.
I just remembered I played with "censorship" on, I wonder if I missed anything.