blitzschnell
lightning-fast
Extremely fast; as fast as lightning.
Mit einer blitzschnellen Reaktion konnte der Torwart den Ball noch abwehren.
With a lightning-fast reaction, the goalkeeper was still able to block the ball.

lightning-fast
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lightning-fast
Extremely fast; as fast as lightning.
Mit einer blitzschnellen Reaktion konnte der Torwart den Ball noch abwehren.
With a lightning-fast reaction, the goalkeeper was still able to block the ball.

The word is a compound of 'Blitz' (lightning) and 'schnell' (fast). So it literally means 'as fast as lightning' and expresses extremely high speed.
Der Gepard ist für seine blitzschnellen Sprints bekannt.
The cheetah is known for its lightning-fast sprints.
'Blitzschnell' (lightning-fast) is very often used as an adverb to describe how an action is carried out. It then remains unchanged in the sentence and refers to the verb.
Er verstand blitzschnell, was er tun musste.
He understood lightning-fast what he had to do.
Common combinations are 'eine blitzschnelle Reaktion' (a lightning-fast reaction), 'eine blitzschnelle Bewegung' (a lightning-fast movement), or verbs such as 'blitzschnell handeln' (to act lightning-fast), 'blitzschnell reagieren' (to react lightning-fast), and 'blitzschnell entscheiden' (to decide lightning-fast).
Dank seiner blitzschnellen Reaktion konnte er einen Unfall verhindern.
Thanks to his lightning-fast reaction, he was able to prevent an accident.
fast / quick
'schnell' and 'blitzschnell' both describe that something happens in a short time or at high speed. 'schnell' is the normal word and fits almost all everyday situations. 'blitzschnell' is stronger and emphasizes that something happens extremely quickly, almost immediately. It sounds more vivid and a bit more figurative. If you only want to say that something happens quickly, 'schnell' is more natural. If you want to highlight the exceptional speed, 'blitzschnell' fits better.

In der Pause beantwortet Simon die Nachricht blitzschnell.
During the break, Simon answers the message lightning-fast.
Vor dem Termin esse ich schnell noch ein Brot in der Küche.
Before the appointment, I quickly eat a slice of bread in the kitchen.