diff --git a/crates/fbuild-core/src/path.rs b/crates/fbuild-core/src/path.rs
index b1711c26..02141dcc 100644
--- a/crates/fbuild-core/src/path.rs
+++ b/crates/fbuild-core/src/path.rs
@@ -407,12 +407,52 @@ pub fn normalize_flags_for_compile_cwd(flags: &[String], cwd: &Path) -> Vec<Stri
     normalized
 }
 
+/// Upper bound on memoized `realpath` results. A build resolves a few hundred
+/// distinct flag paths; the cap only exists so the map cannot grow without
+/// bound in a long-lived daemon.
+const CANONICALIZE_MEMO_CAPACITY: usize = 8192;
+
+/// Memo for [`canonicalize_lexical`]'s successful `realpath` results.
+///
+/// Signature building normalizes every path-bearing flag of every source, so
+/// the *same* include directories are canonicalized once per translation unit.
+/// On ESP32-S3 that is 385 dirs x 46 sources x 2 (`path` + `cwd`) = ~35,000
+/// `realpath` syscalls per build, ~670 ms, on the hot path of every cold *and*
+/// warm build (FastLED/fbuild#1537).
+///
+/// Only full `path.canonicalize()` successes are memoized. The
+/// parent-plus-filename fallback is deliberately not: it is the branch that
+/// answers for paths that do not exist yet, so caching it would keep reporting
+/// a not-yet-created directory as missing for the rest of the process. A
+/// memoized hit is therefore always a path that existed and was resolved once.
+fn canonicalize_memo() -> &'static std::sync::RwLock<std::collections::HashMap<PathBuf, PathBuf>> {
+    static MEMO: std::sync::OnceLock<
+        std::sync::RwLock<std::collections::HashMap<PathBuf, PathBuf>>,
+    > = std::sync::OnceLock::new();
+    MEMO.get_or_init(|| std::sync::RwLock::new(std::collections::HashMap::new()))
+}
+
 /// Canonicalize an existing path (stripping the Windows `\\?\` prefix),
 /// falling back to canonicalizing the parent + rejoining the file name when
 /// the full path does not yet exist. Returns `None` if neither resolves.
 fn canonicalize_lexical(path: &Path) -> Option<PathBuf> {
+    if let Some(hit) = canonicalize_memo()
+        .read()
+        .unwrap_or_else(|e| e.into_inner())
+        .get(path)
+    {
+        return Some(hit.clone());
+    }
     if let Ok(canonical) = path.canonicalize() {
-        return Some(strip_unc_prefix(&canonical));
+        let resolved = strip_unc_prefix(&canonical);
+        let mut memo = canonicalize_memo()
+            .write()
+            .unwrap_or_else(|e| e.into_inner());
+        if memo.len() >= CANONICALIZE_MEMO_CAPACITY {
+            memo.clear();
+        }
+        memo.insert(path.to_path_buf(), resolved.clone());
+        return Some(resolved);
     }
     let parent = path.parent()?.canonicalize().ok()?;
     let joined = match path.file_name() {
@@ -834,6 +874,33 @@ mod tests {
         assert_eq!(arg, "src/sketch/main.cpp");
     }
 
+    #[test]
+    fn canonicalize_lexical_memoizes_resolved_paths() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let dir = tmp.path().join("include");
+        std::fs::create_dir_all(&dir).unwrap();
+        let resolved = dir.canonicalize().unwrap();
+        assert_eq!(canonicalize_lexical(&dir), Some(resolved.clone()));
+        // A second call is served from the memo rather than a fresh realpath.
+        assert_eq!(canonicalize_lexical(&dir), Some(resolved));
+    }
+
+    #[test]
+    fn canonicalize_lexical_does_not_memoize_the_missing_parent_fallback() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let missing = tmp.path().join("later");
+        // First call takes the parent-plus-filename fallback.
+        let before = canonicalize_lexical(&missing).unwrap();
+        assert!(before.ends_with("later"));
+        std::fs::create_dir_all(&missing).unwrap();
+        // Once the directory exists the real path must win, so the fallback
+        // result cannot have been memoized.
+        assert_eq!(
+            canonicalize_lexical(&missing),
+            Some(missing.canonicalize().unwrap())
+        );
+    }
+
     #[test]
     fn path_arg_for_compile_cwd_returns_dot_for_workspace_root() {
         let tmp = tempfile::TempDir::new().unwrap();