Avoiding rebuilds (e.g. of mariadb, the entire Rust chain, etc.)?

[Matthew Brooks <matthewfbrooks@mailbox.org>]

Is there any way to avoid rebuilding stuff like mariadb, the entire Rust chain, etc. unless one of those packages *actually* changes? It seems like every few days every single package needs to rebuild for some reason, including many packages that spend unbelievably long times running tests that will never actually be of use to me, so I'm usually only able to update every couple of weeks since so much constantly needs to be rebuilt and everything takes so long.

It seems to have gotten worse over time as well, as more and more base packages pull in extremely computation-hungry dependency chains. At this very moment, for example, I'm waiting for Rust & co. to compile simply to delete old system generations from the bootloader.

I've got substitutes turned off (primarily because I like the idea of every package being "home built" as it were), but I can't imagine that even the official substitiute servers would be able to keep up with the constant rebuilds my system seems to want unless I've got something horribly misconfigured somewhere.

The definition for the guix service in my config is:

(modify-services
 %base-services
 (guix-service-type
  config =>
  (guix-configuration
   (inherit config)
   (use-substitutes? #f)
   (authorized-keys (list
         (local-file "./laptop-key.pub")
         (local-file "./tablet-key.pub")
         ))
   (substitute-urls '())
   (extra-options '("--gc-keep-derivations=yes" "--gc-keep-outputs=yes" "--no-substitutes"))
   ))
 )

I've virtually zero experience with Scheme or Guile, so maybe there's something obvious I'm missing, or some obvious way to change the behavior, but I've never been able to find anything about it.