How do games speed up and slow down?

[Madtigger]

I've noticed the overhead AutoMap can sometimes cause slowdowns too. I'm just wondering -- why does an "automap" cause such delaysl/lags? It's just a 2D (not 3D?) simple line graphics, isn't it? or maybe not?
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It is a 2D transparent overlay that is rendered above the main image. It slows your machine down a bit because bliting operations (layering of .bmps and .gifs to create a composite image) are computationally expensive.

Now, you might say, 'but I own a GeForce28000 gtxpml, with 2 terrabit 1024 thermo-overdrive graphics piplelines in sterio, and it still runs slow!'

Diablo2 is a hybrid 2D/3D graphics engine, where most of the images you see are 2D pre-rendered sprites, lit using 3D lighting. Because most of the game is 2D, you get a limited performance gain from having a fast 3D card. 3D accelerators are great for computing 3D math, but they largely sit idle when working with a sprite based engine.

Anytime your computer trys to draw a lot of sprites at once, D2 will slow down. Your overhead map is one way to increase the sprite count. Joining a hell baal run with 7 zookeeper necros is another....