Quadratic Hashing in BIP 134

I noticed that the the latest BIP 134 <https://github.com/bitcoin/bips/blob/959fecc15bdad070afa63455468b1dba54655fa6/bip-0134.mediawiki> now supports SIGHASH_SINGLE and friends. However, this support seems to reintroduce some quadratic hashing behavior because it calls <https://github.com/zander/bitcoinclassic/blob/9c688c6d3866890f16a36aaea453e8bdd43c1266/src/script/interpreter.cpp#L1186>SerializePartialTransactionv4 per non-SIGHASH_ALL input <https://github.com/bitcoinclassic/bitcoinclassic/blob/9c688c6d3866890f16a36aaea453e8bdd43c1266/src/script/interpreter.cpp#L1186>. In particular, if each input in a transaction has one SIGHASH_SINGLE CHECKSIG operation then the total amount of hashing done for the transaction will be quadratic in the number of inputs. While amount of hashing is not as severe as with the SIGHASH_ALL case, the amount of hashing done is still non-linear.