What exactly causes lag spikes in maps? And how many avaliable ways there are to optimize a map?

[Aquavine]

So I came back to my old map, and while updating it I noticed a one glaring issue with it. It lags sometimes, but when it does lag, it d o e s. (The worst one that I recorded after all my patches, is 60 to 30/40 fps in a specific section.) And even if it has way better performance after I learned to hide geometry behind thok barriers, It still chokes in some moments. And sometimes I don't know how to make it run better and making it aesthetically pleasing at the same time.

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So does someone know any ways I can optimize any stages, so they'll have better performance? I mostly play on OPENGL and the stage I'm talking about also should be played on this rendering for the best experience, just for additional context. And I already know some optimizing tips, so to avoid repeating I will list them here:
-Merging sectors that are similar to each other, or have the same tags or properties.
-Hiding geometry behind walls so the engine won't have to render the whole stage at the same time.
And sadly, these are all tips I know of at the moment. :C

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And the player can still "optimize" the stage by themselves simply by changing their graphic settings, but If this is what has to be done so your stage can played at barely consistent frame-rate, then this is a problem YOU must fix. Not the other way around.

 

[yfyfyfyfyfy]

the simplest thing is - the less geometry is visible, the less it lags. as you can see cez2 (probably the most technically difficult level in the game) is "divided" into sections and there are no large and open spaces

starting with some version of the game (I don't remember which one) for me personally cez2 stopped lagging at all and gives out ideal 35 or 60+ fps.