Hi Kano.  Maybe what you are missing is that in SegWit blocks, the SegWit transaction signature data is moved into a data structure that older versions of the software don't see nor consider to be a part of the block.  And signature data takes up like 75% of a pre-segwit transaction's size.  Given that we need to forever remember pre-segwit transactions to calculate their txids, but segwit transactions we can eventually throw away the signature data and still calculate the txid... this is the other reason why old transactions are weighted more...  other than that they are also weighted more because they take up more space in the perspective of non-segwit nodes.

Edit: and about the "slow" stuff you are talking about... I think that has more to do with schnorr signatures... "https://en.wikipedia.org/wiki/Schnorr_signature" which are not a part of SegWit.  I don't think CPU time has anything to do with weighting differences.  Correct me if I'm wrong.