Weren’t you looking at compile time all along? So you should have seen the optimization. 
Seriously though, that’s only part of the solution. The fact the comprehension is communicated via a struct, similar to the Rendered struct, arose from the fact we are also tackling runtime. The current implementation can easily nest templates (<%= render ... %>) without traversing or nesting ASTs and we could extend that to comprehensions too. So we are mixing both approaches instead of one or the other.
I am focusing rather on what comes generated on phx.gen.html, and you see both nested templates and comprehensions in there.
It is unavoidable only if you assume it HAS to be done. Most likely it doesn’t and there are probably other ways we can optimize things like forms, looking at its domain in particular and not necessarily at the template engine as a whole. Edit: But of course, doing Vampyre is a great exercise in itself. 
), as 