I plan to write about this in the near future (in fact I'm writing slides for a talk on it this week) – essentially, codegen is more costly because a) you have

I plan to write about this in the near future (in fact I'm writing slides for a talk on it this week) – essentially, codegen is more costly because a) you have to generate the entire program, rather than just modifying the nodes that have changed, b) your code generator needs to know about every single node type, and c) you have to keep track of state (e.g indentation levels).