My slow flow is euphoric, it's like I rap endorphins

from Bad Meets Evil – Fast Lane Lyrics on Rap Genius

Meaning

Endorphins are natural/made drugs that lessen the body’s stress and relaxes him to some point (depending on the dose). Thus, producing an euphoric effect.
So his slow flow is euphoric and goes on to rap faster and faster. If his slow flow is euphoric then the fast flow must be a trip. In the fast lane.

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