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Question about source ports, computer components and framerates

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Which computer components determine how high a framerate you get in the various source ports?

I've always assumed the strength of the processor determined the framerate in a software renderer port and the strength of the graphics card determined the framerate in a gl renderer port. Am I in the ballpark with this assumption or is there more to it?

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With regard to GL source ports the weighting between CPU and GPU varies significantly. Most current GL ports do most of their rendering in what is referred to as "immediate mode" and consequently don't really benefit as much from an uber powerful video card as one would hope.

Given that most GL ports generate new map geometry each and every frame on CPU side, this further skews the CPU / GPU performance weighting toward the CPU.

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exp(x) said:

I imagine main memory speed and latency is also a factor.

This is very important with the Doom software renderer's method of drawing wall columns, which makes extremely poor use of the CPU's cache and causes the CPU to fetch and cache a line of the destination frame buffer from RAM for each pixel of the column the renderer draws. Fortunately there are some tricks to making better usage of the cache such as drawing four interleaved columns at a time to temporary buffer or by drawing in 64x64 blocks but the speed it takes to fetch from RAM is still a bottleneck.

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