träge
sluggish
Slow, without energy or drive; heavy and listless in movement or action.
Nach dem üppigen Mittagessen fühlte er sich träge und müde.
After the large lunch, he felt sluggish and tired.

sluggish / inert
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sluggish
Slow, without energy or drive; heavy and listless in movement or action.
Nach dem üppigen Mittagessen fühlte er sich träge und müde.
After the large lunch, he felt sluggish and tired.

The adjective 'träge' (sluggish/inert) belongs to the word family of 'die Trägheit' (inertia/sluggishness). This noun refers both to the human trait of lack of drive and to the physical principle of inertia.
Die Trägheit der Masse ist ein Grundprinzip der Physik.
The inertia of mass is a basic principle of physics.
You can recognize the meaning of 'träge' (sluggish/inert) from the context. If people, feelings, or slow movements are being discussed, it means 'antriebslos' (lacking drive). In scientific contexts (chemistry, physics) with words like 'Gas' (gas) or 'Stoff' (substance), it means 'does not react'.
Der Fluss bewegt sich hier sehr träge.
The river moves very sluggishly here.
In its everyday meaning, 'träge' (sluggish) is often used to describe a feeling or state. Typical phrases are 'sich träge fühlen' (to feel sluggish), 'träge werden' (to become sluggish), or also for slow, heavy movements, such as 'ein träger Fluss' (a sluggish river).
An heißen Sommertagen fühle ich mich oft besonders träge.
On hot summer days, I often feel especially sluggish.
The meaning 'inert' or 'reaktionsträge' (slow to react) is purely technical language. It is used almost exclusively in the natural sciences, such as physics and chemistry.
Stickstoff ist unter normalen Bedingungen ein relativ träges Element.
Under normal conditions, nitrogen is a relatively inert element.
fast / quick
'schnell' and 'träge' are clear opposites. 'schnell' describes a movement, action, or process with high speed or without a long wait. 'träge' describes someone or something that seems slow, sluggish, and without energy. So 'träge' does not just mean 'slow'; it also suggests a lack of drive or strength. With people, animals, or processes, 'träge' often sounds somewhat negative.

Nach dem langen Mittagessen war er träge und wollte nicht weiterarbeiten.
After the long lunch, he was sluggish and didn't want to keep working.
In der Stadt ist das Fahrrad oft schneller als das Auto.
In the city, a bike is often faster than a car.