perf(startup): Improve startup speed by borrowing from doom-emacs

Shameless borrow various methods of improving Emacs startup time from
doom-emacs:

b0978a4526/docs/faq.org (how-does-doom-start-up-so-quickly)

core/siren-core-init.el

@@ -36,6 +36,7 @@
 
 ;; Continue core stuff
 (require 'siren-core-packages)
+(require 'siren-core-performance)
 (require 'siren-core-env)
 (require 'siren-core-ui)
 (require 'siren-core-editor)

core/siren-core-performance.el

@@ -0,0 +1,20 @@
+;;; siren-core-performance.el --- jimeh's Emacs Siren: Performance tweaks
+
+;;; Commentary:
+
+;; Performance tweaks.
+
+;;; Code:
+
+;; Setup and use gcmh-mode for improved garbage collection.
+(use-package gcmh
+  :hook
+  (emacs-startup . (lambda() (gcmh-mode +1)))
+  (focus-out-hook . gcmh-idle-garbage-collect)
+
+  :custom
+  (gcmh-idle-delay 10)
+  (gcmh-high-cons-threshold 16777216))
+
+(provide 'siren-core-performance)
+;;; siren-core-performance.el ends here

early-init.el

@@ -6,8 +6,22 @@
 
 ;;; Code:
 
+;; Defer garbage collection further back in the startup process
+(setq gc-cons-threshold most-positive-fixnum)
+
 ;; Disable Emacs 27's automatic package.el initialization before the init.el
 ;; file is loaded. I use straight.el instead of package.el.
 (setq package-enable-at-startup nil)
 
+;; Prevent the glimpse of un-styled Emacs by disabling these UI elements early.
+(setq tool-bar-mode nil
+      menu-bar-mode nil)
+(when (fboundp 'set-scroll-bar-mode)
+  (set-scroll-bar-mode nil))
+
+;; Resizing the Emacs frame can be a terribly expensive part of changing the
+;; font. By inhibiting this, we easily halve startup times with fonts that are
+;; larger than the system default.
+(setq frame-inhibit-implied-resize t)
+
 ;;; early-init.el ends here