diff --git a/README.md b/README.md
index efb843f..ff11898 100644
--- a/README.md
+++ b/README.md
@@ -71,7 +71,10 @@ std.debug.print("{f}\n", .{ten}); // 0-306-40615-2
 ## Build options
 
 - `-Dsimd=[bool]` — use SIMD vector operations for computing check
-  digits (default: `true`).
+  digits (default: `false`). It is off because it does not pay: `zig
+  build bench` measures both implementations, and nine or twelve lanes
+  of multiply-and-add do not earn back the cost of widening the digits
+  and assembling the vector.
 
 ## Standards
 
diff --git a/build.zig b/build.zig
index a837ece..8245245 100644
--- a/build.zig
+++ b/build.zig
@@ -7,7 +7,13 @@ pub fn build(b: *std.Build) void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
-    const simd = b.option(bool, "simd", "Use SIMD for computing check digits.") orelse true;
+    // Off by default, because it does not pay. `zig build bench` measures
+    // both: the ISBN-13 checksum comes out about level and the ISBN-10 one
+    // comes out slower, since nine lanes of multiply-and-add do not earn
+    // back the cost of widening the digits and assembling the vector. The
+    // option stays so the two implementations can be compared, and so
+    // anyone whose target says otherwise can turn it on.
+    const simd = b.option(bool, "simd", "Use SIMD for computing check digits.") orelse false;
 
     const options = b.addOptions();
     options.addOption(bool, "simd", simd);