langsam
slow
Describes a movement or process that takes place at low speed.
Die Schnecke ist ein sehr langsames Tier.
The snail is a very slow animal.

slow
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slow
Describes a movement or process that takes place at low speed.
Die Schnecke ist ein sehr langsames Tier.
The snail is a very slow animal.

'Langsam' can stand before a noun as an adjective or describe an action as an adverb. Unlike in many other languages, the form of the word does not change in this use.
Der langsame Verkehr am Morgen ist ärgerlich.
The slow traffic in the morning is annoying.
When 'langsam' describes a process or a change of state (often with the verb 'werden' (to become)), it means “gradually” or “little by little.” It then often stands at the beginning of the sentence.
Langsam verstehe ich, wie das Gerät funktioniert.
I'm gradually understanding how the device works.
In colloquial speech, 'langsam' is used to express growing impatience or annoyance. A very common phrase is 'Langsam reicht es mir!' (I've had enough now!).
Ich warte schon seit einer Stunde auf dich, langsam werde ich ungeduldig.
I've been waiting for you for an hour; I'm starting to get impatient.
fast / quick
'schnell' and 'langsam' are opposites when it comes to speed or duration. 'schnell' is used when a movement, action, or process happens in a short time. 'langsam' is used when something has little speed or happens step by step. The contrast is very direct: a car can drive fast or drive slowly, and a task can be finished quickly or progress slowly.

Nach dem Unfall geht Tom langsam über den Hof.
After the accident, Tom walks slowly across the yard.
Auf der freien Straße fährt der Bus heute ungewöhnlich schnell.
On the empty road, the bus is going unusually fast today.