Performance transitions

[Fims]

What are some ways to transition from mellow songs to heavy and faster songs?

[VomitHatSteve]

Banter's always a good skill to master.
If you're not talking between the songs, the primary ways that you might want to transition are "abruptly" and "gradually".
Abruptly transitioning would mean that as soon as your slow song end, you immediately launch into your fast, loud song. That can sound pretty good.
Gradually transitioning would mean playing something that works as a segue between the two songs. You'd have to write the segue.