快别说了 vs 别说了: tone changed?
快 (quick; hurry) here is a [command/ demand exclamation] We use it in front of a verb or verb phrase to stress the urgency, demands immediate compliance.

(a request) 别说了 = &quot;you've said enough&quot; or &quot;stop talking&quot; (no tone indicated)

(a demand) 快别说了 = shut up (快住口) or 'Stop talking! Now!'


It is similar to 'Now!' in English
Example:
Drop your weapon. (no tone indicated)
Drop your weapon! Now! (commanding/ urgent tone)
Edit:

快住手 = hurry and stop = Stop! Now!

快去救人 = hurry and go save people = Go save people! Now


Note:
快 can also be used as the short form of '就快' (soon) or '快要' (almost)
Example:
(就)快沒彈藥了 - out of ammunition soon
(就)快餓死了 - starving to death soon
快(要)沒彈藥了 - almost out of ammunition
快(要)餓死了 - almost (about to) starve to death