Hi all!

Some of my libraries take a while to load: just adding a (require mylib) to an 
empty #lang racket/base file bumps the compile time from 0.5s to 1.5s, even if 
no bindings from the library are used. After experimenting a bit, it seems that 
the overhead is mainly due to other modules which are transitively required 
(typed/racket, syntax/parse, …).

Assuming that all the files except the main one are already byte-compiled with 
raco make, what are the factors which affect compile times when a library is 
(possibly transitively) required? Note that we're talking about milliseconds 
here, but when a lot of modules are required, these can add up.

1. Number of bindings directly provided?
2. Number of bindings provided by transitively-required modules (including 
those which are not re-provided, and are not in the main file's namespace)
3. Number of files which are transitively required?
4. Size of the fully expanded code (or size of the bytecode)?
5. Compile-time operations, e.g. within a begin-for-syntax in a 
(transitively-)required module?
6. When a module is required at several meta-levels, does it affect the compile 
time significantly, or is it roughly the same cost than requiring it only once?
7. Something else?

Point 5 seems to matter, as compiling a.rkt still prints "Hello" even if b.rkt 
is already compiled:

#lang racket/base
(require "b.rkt")

#lang racket/base
(require (for-syntax racket/base))
(begin-for-syntax (displayln "Hello"))

However, I'm not sure what operations can cause compile-time code to be run in 
this situation. My (possibly incorrect) understanding is that macros are 
executed only once (when expanding the code), but the code to the 
right-hand-side of a (define-syntax foo costly-operation), and the code within 
a (begin-for-syntax …) will both be run each time the module is required.

What I would like is to learn a few "good practices" and gotchas concerning 
compile-time performance.

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