Speed up project root detection by resolving it less often

Projectile can consume a lot of cycles, especially in indirect buffers
or because of project-root-sensitive path segments in the modeline. This
experimental fix should spare you that heartache.

core/core-projects.el

@@ -40,6 +40,16 @@ Emacs.")
   :config
   (projectile-mode +1)
 
+  ;; REVIEW Resolve the project root once, when the file/buffer is opened. This
+  ;;        speeds up projectile's project root resolution by leaps, but does
+  ;;        put you at risk of having a stale project root.
+  (setq-hook! '(after-change-major-mode-hook
+                ;; In case the user saves the file to a new location
+                after-save-hook
+                ;; ...or makes external changes then returns to Emacs
+                focus-in-hook)
+    projectile-project-root (if default-directory (doom-project-root)))
+
   ;; Projectile runs four functions to determine the root (in this order):
   ;;
   ;; + `projectile-root-local' -> checks the `projectile-project-root' variable