diff --git a/.github/workflows/jekyll.yml b/.github/workflows/jekyll.yml
index 4b61249..b9b7510 100644
--- a/.github/workflows/jekyll.yml
+++ b/.github/workflows/jekyll.yml
@@ -40,9 +40,8 @@ jobs:
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1
         with:
-          # Matches .ruby-version. bundler-cache installs the Gemfile (still
-          # the github-pages gem, so the build is byte-for-byte the current
-          # one) and caches gems between runs.
+          # Matches .ruby-version. bundler-cache installs the Gemfile (Jekyll 4
+          # toolchain since issue #81) and caches gems between runs.
           ruby-version: "3.3.11"
           bundler-cache: true
           cache-version: 0
diff --git a/Gemfile b/Gemfile
index 4a2c7ad..6726114 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,11 +1,28 @@
 source "https://rubygems.org"
 
-# gem "jekyll"
-# gem 'kramdown-parser-gfm'
+# These were extracted from the Ruby stdlib in 3.4+; keep them explicit so the
+# build works across Ruby versions.
 gem "csv"
 gem "bigdecimal"
 gem "base64"
-gem "github-pages", group: :jekyll_plugins
+
+# Jekyll 4 toolchain. We dropped the `github-pages` gem, which hard-pinned
+# Jekyll 3.9 (and old liquid/kramdown) and was the main cause of slow builds
+# (issue #81). The site now publishes via a full `bundle exec jekyll build` in
+# GitHub Actions (issue #98), so the legacy GitHub Pages gem environment is no
+# longer required.
+gem "jekyll", "~> 4.3"
+
+# Plugins that the github-pages gem used to auto-enable and that this site
+# relies on. They must now be declared explicitly (also listed in _config.yml
+# `plugins:`).
+group :jekyll_plugins do
+  gem "jekyll-sitemap"          # /sitemap.xml
+  gem "jekyll-relative-links"   # rewrites internal .md links -> .html in source
+  gem "jekyll-include-cache"    # just-the-docs uses {% include_cached %}
+  gem "jekyll-seo-tag"          # just-the-docs head dependency
+end
+
 gem 'wdm', '>= 0.1.0' if Gem.win_platform?
 gem "webrick", "~> 1.7"
-gem "just-the-docs", "0.12.0"
\ No newline at end of file
+gem "just-the-docs", "0.12.0"
diff --git a/_config.yml b/_config.yml
index b02a047..d1a793f 100644
--- a/_config.yml
+++ b/_config.yml
@@ -8,7 +8,10 @@
 # For technical reasons, this file is *NOT* reloaded automatically when you use
 # 'bundle exec jekyll serve'. If you change this file, please restart the server process.
 # remote_theme: pmarsceill/just-the-docs
-remote_theme: just-the-docs/just-the-docs@v0.12.0
+# Use the locally-installed just-the-docs gem (0.12.0, see Gemfile) instead of
+# remote_theme: same theme version, but no network fetch / theme resolution at
+# build time. Part of the Jekyll 4 build-speed work (issue #81).
+theme: just-the-docs
 
 # Fixed at 0.2.8 because of: https://github.com/makeabilitylab/physcomp/issues/9
 # remote_theme: pmarsceill/just-the-docs@v0.2.8
@@ -24,14 +27,17 @@ tagline: Learn Arduino, ESP32, sensors, and more with this open-source interacti
 # See: https://jekyllrb.com/docs/configuration/incremental-regeneration/
 incremental: true
 
-# Google Gemini suggested I try this to reduce build time, which is often 20 secs
-# or longer on my desktop (AMD Ryzen 7 3800X 8-Core Processor 3.90 GHz with 32 GB RAM)
-cache: true 
-
-# Gemini also suggested I disable search for local builds
-# Note: Search is enabled by default (and this follows just-the-docs defaults)
+# Search is enabled by default (follows just-the-docs defaults).
 search_enabled: true
 
+# Sass (jekyll-sass-converter 3.x / dart-sass under Jekyll 4):
+#   quiet_deps: silence the @import / darken() deprecation warnings emitted by
+#               the just-the-docs theme's own SCSS (not our code).
+#   sourcemap:  don't emit .css.map files into _site.
+sass:
+  quiet_deps: true
+  sourcemap: never
+
 # site.url: https://jonfroehlich.github.io/physcomp
 # Make absolute paths work with GitHub pages
 # See: https://stackoverflow.com/a/19173888/388117
@@ -42,12 +48,18 @@ url: "https://makeabilitylab.github.io" # the base hostname & protocol for your
 # For copy button on code
 enable_copy_code_button: true
 
-# Jekyll plugins. jekyll-sitemap generates /sitemap.xml (submittable to search
-# engines) to help indexing. It ships with the github-pages gem and is on the
-# GitHub Pages plugin whitelist, so it works under both the legacy build and the
-# GitHub Actions build (#98).
+# Jekyll plugins. These were previously auto-enabled by the github-pages gem;
+# after moving to Jekyll 4 (issue #81) they are declared explicitly here and in
+# the Gemfile's :jekyll_plugins group.
+#   jekyll-sitemap         -> /sitemap.xml (submittable to search engines)
+#   jekyll-relative-links  -> rewrites internal .md links to .html
+#   jekyll-include-cache   -> just-the-docs uses {% include_cached %}
+#   jekyll-seo-tag         -> just-the-docs head dependency
 plugins:
   - jekyll-sitemap
+  - jekyll-relative-links
+  - jekyll-include-cache
+  - jekyll-seo-tag
 
 # custom site variables
 arduino_github_baseurl: "https://makeabilitylab.github.io/physcomp/"