diff --git a/src/Product.cc b/src/Product.cc
index aaa36c03..5eb18dd6 100644
--- a/src/Product.cc
+++ b/src/Product.cc
@@ -1215,7 +1215,7 @@ static pair<size_t, size_t> compute_offset1_and_limit1(
   }
 }
 
-bool product_is_valid(const string& s, uint8_t domain, uint8_t subdomain) {
+bool product_is_valid_slow(const string& s, uint8_t domain, uint8_t subdomain) {
   uint64_t product = decode_product_str(s);
   if (product == INVALID_PRODUCT) {
     return false;
@@ -1370,7 +1370,7 @@ void product_speed_test(uint64_t seed) {
     time_fast += now() - start;
 
     start = now();
-    bool is_valid_slow = product_is_valid(s, 1, 0xFF);
+    bool is_valid_slow = product_is_valid_slow(s, 1, 0xFF);
     time_slow += now() - start;
 
     if (((z & 0xF) == 0) || is_valid_slow || is_valid_fast) {
@@ -1383,5 +1383,6 @@ void product_speed_test(uint64_t seed) {
 
   fprintf(stderr, "Total time (slow): %" PRId64 " usecs (%" PRIu64 " per product)\n", time_slow, time_slow / count);
   fprintf(stderr, "Total time (fast): %" PRId64 " usecs (%" PRIu64 " per product)\n", time_fast, time_fast / count);
+  fprintf(stderr, "Fast vs. slow speedup: %zux\n", static_cast<size_t>(time_slow / time_fast));
   fprintf(stderr, "Disagreements: %zu\n", num_disagreements);
 }
diff --git a/src/Product.hh b/src/Product.hh
index 5c19258e..78763113 100644
--- a/src/Product.hh
+++ b/src/Product.hh
@@ -5,14 +5,16 @@
 #include <string>
 #include <unordered_map>
 
-bool product_is_valid(
+// product_is_valid_slow is Sega's implementation; product_is_valid_fast
+// produces identical results but is about 7000 times faster.
+bool product_is_valid_slow(
     const std::string& s, uint8_t domain, uint8_t subdomain = 0xFF);
 bool product_is_valid_fast(
     const std::string& s, uint8_t domain, uint8_t subdomain = 0xFF);
 bool product_is_valid_fast(
-    uint32_t product, uint8_t domain, uint8_t subdomain);
+    uint32_t product, uint8_t domain, uint8_t subdomain = 0xFF);
 bool decoded_product_is_valid_fast(
-    uint32_t product, uint8_t domain, uint8_t subdomain);
+    uint32_t product, uint8_t domain, uint8_t subdomain = 0xFF);
 
 std::string generate_product(uint8_t domain, uint8_t subdomain = 0xFF);
 std::unordered_map<uint32_t, std::string> generate_all_products(uint8_t domain = 0xFF, uint8_t subdomain = 0xFF);