diff --git a/bench.py b/bench.py
index df804ea..6e85a46 100644
--- a/bench.py
+++ b/bench.py
@@ -27,6 +27,7 @@
 # name -> (function name, keyword arguments, input preparation)
 OPERATIONS = {
     "b64encode": ("b64encode", {}, lambda raw: raw),
+    "b64encode(altchars)": ("b64encode", {"altchars": b"*!"}, lambda raw: raw),
     "b64decode": ("b64decode", {}, base64.b64encode),
     "b64decode(validate=True)": ("b64decode", {"validate": True}, base64.b64encode),
     "urlsafe_b64encode": ("urlsafe_b64encode", {}, lambda raw: raw),
diff --git a/src/fastpybase64.cpp b/src/fastpybase64.cpp
index 0c85540..c2d9ceb 100644
--- a/src/fastpybase64.cpp
+++ b/src/fastpybase64.cpp
@@ -215,12 +215,12 @@ PyObject *encode_with_altchars(const InputView &input, const char altchars[2]) {
   }
   char *out = PyBytes_AS_STRING(result);
   Py_ssize_t length = PyBytes_GET_SIZE(result);
+  const char plus = altchars[0];
+  const char slash = altchars[1];
+  // Store every byte so the compiler can vectorize the substitution.
   for (Py_ssize_t i = 0; i < length; i++) {
-    if (out[i] == '+') {
-      out[i] = altchars[0];
-    } else if (out[i] == '/') {
-      out[i] = altchars[1];
-    }
+    const char c = out[i];
+    out[i] = c == '+' ? plus : (c == '/' ? slash : c);
   }
   return result;
 }