Typically used attributively; no comparative or superlative forms in standard usage.
Translations
The new processor offers lightning-fast performance.
Le nouveau processeur offre des performances ultrarapides.
rapide comme l’éclairLittéral, plus imagé et expressif. Extremely fast; very rapid. The new processor offers lightning-fast performance.
Informal use meaning "very rapidly".
Translations
She typed lightning-fast to meet the deadline.
Elle a tapé à toute vitesse pour respecter la date limite.
à la vitesse de l’éclairExpression soutenue, très imagée. With extreme speed; very rapidly. She typed lightning-fast to meet the deadline.