rasant
fast
Very fast, happening at high speed or within a short time.
Das Auto beschleunigte mit rasanter Geschwindigkeit.
The car accelerated at a fast speed.

rapid / fast
Loading your saved state.
fast
Very fast, happening at high speed or within a short time.
Das Auto beschleunigte mit rasanter Geschwindigkeit.
The car accelerated at a fast speed.

'Rasant' (rapid/fast) is an expressive word that is often used in the press, advertising, or sports commentary to describe speed or change in a dynamic and impressive way.
Die Zeitung berichtete über den rasanten Aufstieg des jungen Politikers.
The newspaper reported on the young politician's rapid rise.
The meaning depends on the context. In connection with movement ('Fahrt' — ride/drive, 'Tempo' — pace/speed), 'rasant' means very fast. In connection with abstract nouns ('Entwicklung' — development, 'Wandel' — change), it means dynamic and impressive.
Der Skifahrer legte eine rasante Abfahrt hin.
The skier made a fast descent.
When it is about speed, 'rasant' is often combined with nouns such as 'Fahrt' (ride/drive), 'Tempo' (pace/speed), or 'Geschwindigkeit' (speed). It can also be used as an adverb, e.g. 'rasant zunehmen' (to increase rapidly).
Die Zahl der Anmeldungen ist rasant gestiegen.
The number of registrations has risen rapidly.
To describe a dynamic change, 'rasant' is frequently used with nouns such as 'Entwicklung' (development), 'Anstieg' (increase), 'Wachstum' (growth), or 'Wandel' (change).
Das Unternehmen erlebte ein rasantes Wachstum.
The company experienced rapid growth.
fast / quick
'schnell' and 'rasant' both describe high speed or a short amount of time. 'schnell' is neutral and works in many different situations. 'rasant' is stronger and sounds more dynamic. It fits well when something happens very quickly, noticeably, or impressively, for example a drive, a game, or a development. For normal everyday actions, 'schnell' is usually more natural. 'rasant' is used more when you want to particularly emphasize the speed.

Die Technik entwickelt sich rasant, deshalb müssen Firmen ständig dazulernen.
Technology is developing rapidly, so companies have to keep learning all the time.
Beim Training merkt Nina, dass sie heute schneller läuft als letzte Woche.
During training, Nina notices that she is running faster today than last week.