diff --git a/src/at/internal/ascii.gleam b/src/at/internal/ascii.gleam
new file mode 100644
index 0000000..0a17a31
--- /dev/null
+++ b/src/at/internal/ascii.gleam
@@ -0,0 +1,6 @@
+/// A more-efficient version of `string.slice` that relies on the input string
+/// being made up of only ASCII characters.
+/// 
+@external(erlang, "binary", "part")
+@external(javascript, "./ascii_ffi.mjs", "slice")
+pub fn slice(string: String, at from: Int, length length: Int) -> String
diff --git a/src/at/internal/ascii_ffi.mjs b/src/at/internal/ascii_ffi.mjs
new file mode 100644
index 0000000..5540875
--- /dev/null
+++ b/src/at/internal/ascii_ffi.mjs
@@ -0,0 +1,3 @@
+export function slice(string, from, length) {
+  return string.slice(from, from + length);
+}