I had written quite a few flows before I found out how slow they run when variables are accessed inside of an Apply-to-each loop. I understand why they are slower - because of locks, but had no idea how slow. I have since switched to Compose where I can, but it might be good if those creating Flows were made aware of this via a warning. Some of my flows run in 1/10th the time once they are rewritten with Compose.
I will also mention that I love the hack that lets me use a Compose outside of an Apply-to-each block to gather an array of a Compose element inside the block. It's not obvious and would be a good thing to document - or even better - make accessible without having to first create the Compose inside the block and copy the contents to a Compose outside the block.