Dont worry psychoak, I figured it out!
The solution is actually partly texture based.
The game knows not to render black/near-black textures. Black shows up as clear/transparent in the game. This is the key to Advent's instant beams. I'll explain:
Beams in the game are made up of 2 parts: Core and Glow. Supposedly one is meant to be the middle of the beam, and the other the...well... glow of the beam. However, glows can simply be applied to the textures themselves, so it seems a bit redundant to have glow right?
Thats what I thought at first. Until I looked closer at all the beams. Advent have their ACTUAL beams named "PsiCapitalBeam_Glow.dds". Their regular CORE texture (PsiCapitalBeam_Core.dds)? Its black/grayish. It doesnt render in the game.
-Okay, so Advent beams appear instantly because their TEXTURE is named to the glow category? Not quite.
The real defining factor for beams fading in or appearing instantly is in the .entity files for the ship, under their weapon effects for beams.
Under a beam's stats you're allowed to set their effects. There are 2 parts here, beamGlowTextureName, and beamCoreTextureName, the same way as stated above. Under these conditions are the key to instant beams: Any texture called to from beamGlowTextureName will appear INSTANTLY in game. Textures called to from beamCoreTextureName will fade in.
beamGlowTextureName=Instant Beam Texture
beamCoreTextureName=Fading Beam Texture
You can see this displayed with TEC Starbase/Titan Beams as well as Vasari Planet bombing beams. Their .entity files have their main beam textures set to CORE, meaning they fade. Further, their textures are named "TechStarbaseBeam_Core" and "PhaseCapitalBombingBeam_Core", with their respective glow textures being nearly black.
-Okay, so how do I make TEC/Vasari Beams instant like Advent's?
There are 2 ways to do this.
1. The simpliest solution is simply to swap the names of the glow and core textures themselves. This way, any ship that uses fading beams will have the main beam texture appear instantly.
2. If you want specific ships to use fading beams and others instant beams without modifying the beams for the whole faction, you can go into each ships' .entity file and swap the textures that are called to under beamGlowTextureName and beamCoreTextureName in the ships beam weapon sections.
Advent beams appear instantly because the .entity files have their beams appear under beamGlowTextureName.
To get instant beams, use the texture you want under beamGlowTextureName in a ship's .entity file.