schnell
fast
Describes a movement, process, or action that happens at high speed or in a short time.
Ein Gepard ist ein sehr schnelles Tier.
A cheetah is a very fast animal.

fast / quick
Loading your saved state.
fast
Describes a movement, process, or action that happens at high speed or in a short time.
Ein Gepard ist ein sehr schnelles Tier.
A cheetah is a very fast animal.

'Schnell' (fast/quick) can be an adjective before a noun (and is then declined) or an adverb describing a verb (and then remains unchanged).
Der ICE ist ein schneller Zug.
The ICE is a fast train.
In everyday speech, 'schnell' (quickly) often does not mean 'at high speed', but rather 'briefly' or 'for a moment'. It expresses that something is uncomplicated and does not take long.
Kannst du mir mal schnell den Salzstreuer geben?
Could you quickly pass me the salt shaker?
As an adjective, 'schnell' (quick/fast) is often combined with abstract nouns to express efficiency or a prompt reaction. Typical examples are 'eine schnelle Lösung' (a quick solution), 'eine schnelle Antwort' (a quick reply), or 'schnelle Hilfe' (quick help).
Vielen Dank für deine schnelle Antwort auf meine E-Mail.
Thank you very much for your quick reply to my email.
immediate / same
'gleich' and 'schnell' can both appear in situations where something should not take long. But the difference is important: 'gleich' says when something happens, namely in a very short moment or very soon. 'schnell' says how something happens, namely at high speed or quickly. They are easy to confuse because both are connected with a short amount of time. If you mean the point in time, use 'gleich'. If you mean the speed, use 'schnell'.

Kannst du bitte schnell die Tür schließen?
Could you please close the door quickly?
Frau Lange kommt gleich zurück und erklärt den nächsten Schritt.
Ms. Lange will be back in a moment and explain the next step.