comment explaining new wallet format for key/wkey entries

src/walletdb.cpp

@@ -317,6 +317,12 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
                 ssValue >> wkey;
                 pkey = wkey.vchPrivKey;
             }
+            
+            // Old wallets store keys as "key" [pubkey] => [privkey]
+            // ... which was slow for wallets with lots of keys, because the public key is re-derived from the private key
+            // using EC operations as a checksum.
+            // Newer wallets store keys as "key"[pubkey] => [privkey][hash(pubkey,privkey)], which is much faster while
+            // remaining backwards-compatible.
             try
             {
                 ssValue >> hash;