diff --git a/benchmark/string_concat.yml b/benchmark/string_concat.yml
index 656bcd1cd79889..0ff1dc25b63351 100644
--- a/benchmark/string_concat.yml
+++ b/benchmark/string_concat.yml
@@ -1,6 +1,7 @@
 prelude: |
   CHUNK = "a" * 64
   BCHUNK = "a".b * 64
+  GC.disable # GC causes a lot of variance
 benchmark:
   binary_concat_utf8: |
     buffer = String.new(capacity: 4096, encoding: Encoding::BINARY)
@@ -11,7 +12,7 @@ benchmark:
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
-    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
+    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
   binary_concat_binary: |
     buffer = String.new(capacity: 4096, encoding: Encoding::BINARY)
     buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
@@ -21,7 +22,7 @@ benchmark:
     buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
     buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
     buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
-    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
+    buffer << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK << BCHUNK
   utf8_concat_utf8: |
     buffer = String.new(capacity: 4096, encoding: Encoding::UTF_8)
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
@@ -31,4 +32,4 @@ benchmark:
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
     buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
-    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
+    buffer << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK << CHUNK
diff --git a/string.c b/string.c
index 8f519da7d10447..c726adb2c38e79 100644
--- a/string.c
+++ b/string.c
@@ -150,7 +150,21 @@ VALUE rb_cSymbol;
     }\
 } while (0)
 
-#define TERM_LEN(str) rb_enc_mbminlen(rb_enc_get(str))
+static inline bool
+str_enc_fastpath(VALUE str)
+{
+    // The overwhelming majority of strings are in one of these 3 encodings.
+    switch (ENCODING_GET_INLINED(str)) {
+      case ENCINDEX_ASCII_8BIT:
+      case ENCINDEX_UTF_8:
+      case ENCINDEX_US_ASCII:
+        return true;
+      default:
+        return false;
+    }
+}
+
+#define TERM_LEN(str) (str_enc_fastpath(str) ? 1 : rb_enc_mbminlen(rb_enc_from_index(ENCODING_GET(str))))
 #define TERM_FILL(ptr, termlen) do {\
     char *const term_fill_ptr = (ptr);\
     const int term_fill_len = (termlen);\
@@ -3311,20 +3325,6 @@ rb_str_buf_cat_ascii(VALUE str, const char *ptr)
     }
 }
 
-static inline bool
-str_enc_fastpath(VALUE str)
-{
-    // The overwhelming majority of strings are in one of these 3 encodings.
-    switch (ENCODING_GET_INLINED(str)) {
-      case ENCINDEX_ASCII_8BIT:
-      case ENCINDEX_UTF_8:
-      case ENCINDEX_US_ASCII:
-        return true;
-      default:
-        return false;
-    }
-}
-
 VALUE
 rb_str_buf_append(VALUE str, VALUE str2)
 {