diff --git a/quest/m0/checks/README.md b/quest/m0/checks/README.md
index 21bce22ff..b42eb97cc 100644
--- a/quest/m0/checks/README.md
+++ b/quest/m0/checks/README.md
@@ -18,7 +18,6 @@ Each child reports its CPU saved, cold and warm, against `origin/main`.
 
 ## Required
 
-- [A cheaper differential](/quest/m0/checks/differential.md) - fixtures' wasm builds at opt-level 1 under single-threaded Node, and `stable_names` reuses `lang`'s translation
 - [Checks by the diff](/quest/m0/checks/dispatch.md) - `just ci` runs only the projects a change touches, as moq's `sh/dispatch.sh` does, with a nightly `just gate`
 - [A warm gate](/quest/m0/checks/cache.md) - the land gate builds through mbx, as moq's does
 - [Stable names on a small fixture](/quest/m0/checks/stable-names.md) - `stable_names` runs on a small fixture with generic instances and trait impls
diff --git a/quest/m0/checks/differential.md b/quest/m0/checks/differential.md
deleted file mode 100644
index fd88f5db2..000000000
--- a/quest/m0/checks/differential.md
+++ /dev/null
@@ -1,18 +0,0 @@
-# [S] A cheaper differential
-
-## Goal
-
-The ts differential fixtures run their native wasm32 side built at opt-level 1, under
-`node --single-threaded`, and `stable_names` reuses the `lang` test's translation as its
-base, with every case still compared.
-
-## Plan
-
-Measured by the 2026-11-01 checks audit on nixos2: `lang`'s differential spends about 590 of
-its 872 CPU-seconds running a 212 MB debug wasm in V8. At opt-level 1 it takes 164, with 0
-differences across 709k cases. Overflow checks and debug assertions stay on, so panics and
-overflow still match. Apply it to every differential fixture. `stable_names`'s base config
-equals `lang`'s roots, so its first translation repeats the `lang` test's (about 150
-CPU-seconds).
-
-Public API: none.
diff --git a/quest/m0/checks/stable-names.md b/quest/m0/checks/stable-names.md
index 35feda6c7..3eb63a0da 100644
--- a/quest/m0/checks/stable-names.md
+++ b/quest/m0/checks/stable-names.md
@@ -9,7 +9,7 @@ has generic instances and trait impls, instead of translating all of `lang`.
 
 Decided with the maintainer on 2026-11-01: it caught 3 failures nothing else did, so it
 stays in the gate, but on a fixture that costs a fraction of `lang`'s (about 300
-CPU-seconds saved after [the reuse](/quest/m0/checks/differential.md)). Cover each naming
+CPU-seconds saved now that it reuses `lang`'s translation). Cover each naming
 rule it relies on in `lang` today.
 
 Public API: none.
diff --git a/ts/Cargo.toml b/ts/Cargo.toml
index 063f12f3c..3909e04b8 100644
--- a/ts/Cargo.toml
+++ b/ts/Cargo.toml
@@ -40,6 +40,13 @@ toml = "1.1.8"
 [lints]
 workspace = true
 
+# The differential tests' native side: optimized, since V8 runs a debug build's wasm several
+# times slower, with dev's overflow checks and debug assertions, so its panics match the
+# translation's.
+[profile.differential]
+inherits = "dev"
+opt-level = 1
+
 # The size-optimized profile moq's WASM shipped with: the benchmark's WASM route.
 [profile.wasm-release]
 inherits = "release"
diff --git a/ts/tests/fixtures.rs b/ts/tests/fixtures.rs
index 2f0b2d825..b3ed2a2eb 100644
--- a/ts/tests/fixtures.rs
+++ b/ts/tests/fixtures.rs
@@ -5,9 +5,13 @@
 
 use std::path::{Path, PathBuf};
 use std::process::{Command, Output};
+use std::sync::OnceLock;
 
 const TARGET: &str = "wasm32-unknown-unknown";
 
+/// The native side's profile, from `Cargo.toml`.
+const PROFILE: &str = "differential";
+
 fn manifest_dir() -> PathBuf {
 	PathBuf::from(env!("CARGO_MANIFEST_DIR"))
 }
@@ -161,6 +165,8 @@ fn differential(package: &str, build: &[&str], out: &Path, entry: &str, cases: u
 			package,
 			"--target",
 			TARGET,
+			"--profile",
+			PROFILE,
 			"--target-dir",
 		])
 		.arg(&target_dir)
@@ -173,10 +179,13 @@ fn differential(package: &str, build: &[&str], out: &Path, entry: &str, cases: u
 	assert!(status.success(), "building {package} for {TARGET} failed");
 	let wasm = target_dir
 		.join(TARGET)
-		.join("debug")
+		.join(PROFILE)
 		.join(format!("{}.wasm", package.replace('-', "_")));
 	std::fs::write(out.join("package.json"), "{ \"type\": \"module\" }\n").unwrap();
+	// One thread: the cases run one at a time, and V8's background compilers and
+	// collectors would spend more CPU than they save.
 	let output = Command::new("node")
+		.arg("--single-threaded")
 		.arg(manifest_dir().join("tests").join("differential.ts"))
 		.arg(&wasm)
 		.arg(out.join(entry))
@@ -284,50 +293,44 @@ fn shake(out: &Path) {
 /// text and order, though translation reaches it in another order.
 #[test]
 fn source_order() {
-	added_root(
-		"codec",
-		r#"roots = ["harness::run", "bench::run"]"#,
-		r#"roots = ["harness::run"]"#,
-		// First, so that translation reaches what both share in another order.
-		r#"roots = ["bench::run", "harness::run"]"#,
-	);
+	let roots = r#"roots = ["harness::run", "bench::run"]"#;
+	let base = rooted("codec", roots, "base", r#"roots = ["harness::run"]"#);
+	// First, so that translation reaches what both share in another order.
+	let added = r#"roots = ["bench::run", "harness::run"]"#;
+	kept(&base, &rooted("codec", roots, "added", added), added);
 }
 
 /// Adding a root that reaches another instance of a generic function, or another trait's
 /// impl of a method, renames nothing the old roots reach.
 #[test]
 fn stable_names() {
+	// `lang`'s own roots, so its translation is the base.
 	let roots = r#"roots = ["harness::run", "harness::op", "fmt::Fault"]"#;
-	added_root(
-		"lang",
-		roots,
-		roots,
-		r#"roots = ["fmt::added", "harness::run", "harness::op", "fmt::Fault"]"#,
-	);
+	let added = r#"roots = ["fmt::added", "harness::run", "harness::op", "fmt::Fault"]"#;
+	kept(&sources(lang_out()), &rooted("lang", roots, "added", added), added);
 }
 
-/// Translate fixture `name` with its config's `roots` line replaced by `base`, then by
-/// `added`, and check that every line of the first output stays, in order, in the second.
-fn added_root(name: &str, roots: &str, base: &str, added: &str) {
+/// Translate fixture `name` into `file` with its config's `roots` line replaced by `with`.
+fn rooted(name: &str, roots: &str, file: &str, with: &str) -> Vec<(PathBuf, String)> {
 	let config = std::fs::read_to_string(fixture(name).join("ts.toml")).unwrap();
 	assert!(config.contains(roots), "{name}'s roots changed: update this test");
 	let dir = scratch().join(name).join("order");
 	std::fs::create_dir_all(&dir).unwrap();
-	let run = |file: &str, with: &str| {
-		let path = dir.join(format!("{file}.toml"));
-		std::fs::write(&path, config.replace(roots, with)).unwrap();
-		let out = dir.join(file);
-		let output = driver(name, &path, &["--out".as_ref(), out.as_ref()], "order");
-		assert!(
-			output.status.success(),
-			"translating {name} with {with} failed:\n{}",
-			String::from_utf8_lossy(&output.stderr)
-		);
-		sources(&out)
-	};
-	let base = run("base", base);
-	let more = run("added", added);
-	for (rel, text) in &base {
+	let path = dir.join(format!("{file}.toml"));
+	std::fs::write(&path, config.replace(roots, with)).unwrap();
+	let out = dir.join(file);
+	let output = driver(name, &path, &["--out".as_ref(), out.as_ref()], "order");
+	assert!(
+		output.status.success(),
+		"translating {name} with {with} failed:\n{}",
+		String::from_utf8_lossy(&output.stderr)
+	);
+	sources(&out)
+}
+
+/// Check that every line of `base` stays, in order, in `more`, its translation once `added`.
+fn kept(base: &[(PathBuf, String)], more: &[(PathBuf, String)], added: &str) {
+	for (rel, text) in base {
 		let more = more.iter().find(|(r, _)| r == rel).map(|(_, text)| text);
 		let more = more.unwrap_or_else(|| panic!("{} is gone once {added}", rel.display()));
 		let mut rest = lines(more).into_iter();
@@ -358,9 +361,15 @@ fn lines(text: &str) -> Vec<String> {
 
 #[test]
 fn lang() {
-	let out = translated("lang");
-	snapshot("lang", &out);
-	lang_differential(&out);
+	let out = lang_out();
+	snapshot("lang", out);
+	lang_differential(out);
+}
+
+/// `lang`'s translation, which `stable_names` shares rather than translating it again.
+fn lang_out() -> &'static Path {
+	static OUT: OnceLock<PathBuf> = OnceLock::new();
+	OUT.get_or_init(|| translated("lang"))
 }
 
 fn lang_differential(out: &Path) {
@@ -630,7 +639,7 @@ fn sysroot() {
 
 /// Selects the fixture's own getrandom backend, which `rand::rng()` seeds from natively:
 /// wasm32 has no OS to read. Only getrandom's flags change, so nothing else rebuilds.
-const GETRANDOM: &str = r#"profile.dev.package.getrandom.rustflags=["--cfg", 'getrandom_backend="custom"']"#;
+const GETRANDOM: &str = r#"profile.differential.package.getrandom.rustflags=["--cfg", 'getrandom_backend="custom"']"#;
 
 /// `bytes`, `rand`, and `tracing`, which translation maps without a binding or config:
 /// it picks getrandom's backend itself, as a browser's binary would.