A lot of CSGO stuff tends to not even bother with normal maps.. And from my experience, I find that Source does some pretty weird shit with normal maps, they never seem to display properly 
We use a lot more normal maps in Insurgency then Valve usualy does for its games. It's not a big performance issue in my experience, tho might be for you in comparison with all the other cs:go maps. People on potatoes might have to scale down their texture quality settings only for your map.
What kills performance in source more then anything is overdraw from alphamapped textures aka tons of foliage, overlapping decals etc. It can bring even a highend rig to its knees while it doesn't look even remotly as good as Cryengine with tons more foliage. Anything else is not such a big problem. You can have lots of geometry in your maps. Valve doesn't even bother about lod's anymore for that reason.
I'm kinda glad they don't bother with normal maps.. unless it's something important like characters. I'd rather see more geo and a pretty diffuse than a fugly normal map.
They add normalmaps where they are actually needed.
Has more to do with their philosophy that the games must run on the worst possible hardware and source's inability to make normals pop properly in many lighting scenarios aynway. Ther's a lot of stuff in cs:go or the portal games that could look better even in that engine but it's a very deliberate choice by Valve to keep it more lowfi. They know people will buy their games for the gameplay and they couldn't compete with the visuals of highenend Cryengine or Unreal games even if they tried.
That could actualy have a backlash. I see that happening now in the reviews for Insurgency. We tried to push the engine a little more but now it doesn't get compared visualy to other indie titles like Rust or DayZ who have similarely small art teams or even CS:GO but to AAA titles and of course can't win on that front. It's a bit of an uncomfy spot inbetween two chairs.
The settings for the Phong shading make a lot of difference when trying to get normal maps to pop in Source, I find. A lot of things just don't need extra maps when similar effects can be accomplished with less (or maybe more polygons, but this doesn't really matter in CSGO anyway; look at the budget when a bunch of expensive player models are on screen for example). As Steppen mentions, translucency overdraw is the big performance killer more than anything else.
You shouldn't have to fight the battle of performance vs graphical fidelity unless you're slapping normal maps on everything and using huge materials and expecting this to make everything look great, in which case you should probably rethink how you are using the assets and whether they really warrant the extra maps (or resolution) in the first place. You can get into a little trial-and-error when it comes to optimizing levels and visibility, but managing performance based on the assets you are using should be pretty straightforward.
If you really run into performance issues and you have a lot of materials that require normal maps consider using Source's self-shadowing bump maps which are slightly cheaper and sometimes provide a more suitable effect.
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