diff --git a/native/philomena/Cargo.lock b/native/philomena/Cargo.lock
index d2481f12..54aa5826 100644
--- a/native/philomena/Cargo.lock
+++ b/native/philomena/Cargo.lock
@@ -357,9 +357,9 @@ dependencies = [
 
 [[package]]
 name = "once_cell"
-version = "1.19.0"
+version = "1.20.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3fdb12b2476b595f9358c5161aa467c2438859caa136dec86c26fdd2efe17b92"
+checksum = "1261fe7e33c73b354eab43b1273a57c8f967d0391e80353e51f764ac02cf6775"
 
 [[package]]
 name = "percent-encoding"
@@ -375,6 +375,7 @@ dependencies = [
  "comrak",
  "http",
  "jemallocator",
+ "once_cell",
  "regex",
  "ring",
  "rustler",
diff --git a/native/philomena/Cargo.toml b/native/philomena/Cargo.toml
index 4ff4467d..e6bba775 100644
--- a/native/philomena/Cargo.toml
+++ b/native/philomena/Cargo.toml
@@ -14,6 +14,7 @@ base64 = "0.21"
 comrak = { git = "https://github.com/philomena-dev/comrak", branch = "philomena-0.29.2", default-features = false }
 http = "0.2"
 jemallocator = { version = "0.5.0", features = ["disable_initial_exec_tls"] }
+once_cell = "1.20"
 regex = "1"
 ring = "0.16"
 rustler = "0.35"
diff --git a/native/philomena/src/domains.rs b/native/philomena/src/domains.rs
index c5626c12..9bcacfc9 100644
--- a/native/philomena/src/domains.rs
+++ b/native/philomena/src/domains.rs
@@ -1,25 +1,29 @@
-use http::Uri;
-use regex::Regex;
+use std::collections::BTreeSet;
 use std::env;
 
-pub fn get() -> Option<Vec<String>> {
+use http::Uri;
+use once_cell::sync::Lazy;
+use regex::Regex;
+
+pub type DomainSet = BTreeSet<String>;
+
+static DOMAINS: Lazy<Option<DomainSet>> = Lazy::new(|| {
     if let Ok(domains) = env::var("SITE_DOMAINS") {
-        return Some(
-            domains
-                .split(',')
-                .map(|s| s.to_string())
-                .collect::<Vec<String>>(),
-        );
+        return Some(domains.split(',').map(|s| s.to_string()).collect());
     }
 
     None
+});
+
+pub fn get() -> &'static Option<DomainSet> {
+    &DOMAINS
 }
 
-pub fn relativize(domains: &[String], url: &str) -> Option<String> {
+pub fn relativize(domains: &DomainSet, url: &str) -> Option<String> {
     let uri = url.parse::<Uri>().ok()?;
 
     if let Some(a) = uri.authority() {
-        if domains.contains(&a.host().to_string()) {
+        if domains.contains(a.host()) {
             if let Ok(re) = Regex::new(&format!(r#"^http(s)?://({})"#, regex::escape(a.host()))) {
                 return Some(re.replace(url, "").into());
             }
@@ -29,6 +33,6 @@ pub fn relativize(domains: &[String], url: &str) -> Option<String> {
     Some(url.into())
 }
 
-pub fn relativize_careful(domains: &[String], url: &str) -> String {
+pub fn relativize_careful(domains: &DomainSet, url: &str) -> String {
     relativize(domains, url).unwrap_or_else(|| url.into())
 }
diff --git a/native/philomena/src/markdown.rs b/native/philomena/src/markdown.rs
index 778deb95..fbd846ac 100644
--- a/native/philomena/src/markdown.rs
+++ b/native/philomena/src/markdown.rs
@@ -29,8 +29,8 @@ pub fn common_options() -> Options {
     options.extension.image_url_rewriter = Some(Arc::new(|url: &str| camo::image_url_careful(url)));
 
     if let Some(domains) = domains::get() {
-        options.extension.link_url_rewriter = Some(Arc::new(move |url: &str| {
-            domains::relativize_careful(&domains, url)
+        options.extension.link_url_rewriter = Some(Arc::new(|url: &str| {
+            domains::relativize_careful(domains, url)
         }));
     }
 
diff --git a/native/philomena/src/tests.rs b/native/philomena/src/tests.rs
index 84418ed4..547f0f86 100644
--- a/native/philomena/src/tests.rs
+++ b/native/philomena/src/tests.rs
@@ -236,8 +236,8 @@ fn image_mention_line_start() {
 
 #[test]
 fn auto_relative_links() {
-    let domains: Vec<String> = vec!["example.com".into()];
-    let f = Arc::new(move |url: &str| domains::relativize_careful(&domains, url));
+    let domains = Arc::new(vec!["example.com".into()].into_iter().collect());
+    let f = Arc::new(move |url: &str| domains::relativize_careful(&*domains, url));
 
     html_opts_i(
         "[some link text](https://example.com/some/path)",