I made a torture machine for the biters in my base and when i look at it it kills the performance so it'd be cool if there was at least an option to speed up the fade time; also when I'm on the main menu anything in the simulations that has train related entities brings me down to 15fps

Two question first:
Which graphics card do you have?
Does your torture machine have also trains?

i have an rtx 3070 and no the trains in the menu simulations are lagging my game

The lag is due to the fact that with all my mod packages enabled, you have too little V-RAM (20 GB are required) and your graphics card has to load textures from the swap space. In the case of the railway and the enemies, the textures are particularly large and delay this process.
I recommend that you disable the Elevates Rail Mod, the Base Game Railway Mod and all Enemy Mods. Unfortunately, I can't offer you a compromise because V-RAM is the price you have to pay for HD textures.

Regarding the fading of the biter corpses, there are not only the biter corpses (Nauvis Enemies Mod), but also the decay ground textures (Base Game Base Mod) and decay particles (Base Game Decoratives) that play a role.
I want to keep the mod as simple as possible without any cross mod options and dependencies between my Mods.

ok

Maybe take a look at this mod: https://mods.factorio.com/mod/ups-fps