Visual Cost

Visual cost is a the combined memory/CPU/GPU use of a specific skill/item/effect in the game.
Typical costs:
Lowest - passive skills, static changes in character appearance(equip)
Low - simple animated textures(terrain,buff/debuff overlays)
Medium - dynamic effects(particles, attacks, simple projectiles), these stack with movement and size.
High - Auras(especially with complex animations), Area-of-effect spells,Complex projectiles(splitting, arcing,growing)
Highest - moving animations of anything above(e.g. long complex skills with area of effect animations). Easily stack to ruin performance.

The formula is basically: movement*complexity*size
To reduce resource use, these guidelines are followed:
Complex+Moving must be small(e.g. low-impact projectiles)
Complex+Large must be static(not moving or rarely moving)
Moving+Large must be simple(e.g. simple behavior).

The usual idea(that doesn't work) that limits use of high-resource effects is to limit their simultaneous appearance(large cooldowns, high cost), but if they have utility players will build their character to use them as often as possible. Planning in advance to reduce performance impact will provide much better multiplayer experience and reduce latency costs associated with complex skills.

Highest cost
Complex Aura with animated components/effects