Optimization/Level Design

Areas

Leaks

There's no excuse! Leaks will invalidate almost all of your efforts elsewhere.

Visibility

Reducing the number of surfaces and objects that are drawn in the first place. This is the largest and most important area.

Physics

Various tricks to avoid overloading the CPU with physics calculations.

Materials

Correct material choices will allow your map to scale down its demands on slower computers.

Lighting

Performance and file-size optimisation.

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Commands

There are plenty more than those listed below — find them in their relevant articles. Note that sv_cheats must be enabled for most to work.

cl_showfps

A simple output of framerate. 1 is real-time, 2 is averaged over the past second.

showbudget

A panel which displays how your computer is spending its budget for each frame. It's the premier tool for working out exactly what's sucking up performance in your map.

It's invoked with +showbudget and -showbudget, which means that you can bind the former to a key (i.e. bind <key> +showbudget) and it will only appear when you hold that button down.

mat_wireframe

A console variable that lets you see through walls. This way, you can see exactly what is being drawn - for reasons described in the visibility optimisation article, often more than you might think is sensible.

There are three wireframe modes, 1 through to 3, which display the information you need with progressively fewer lines.