Not all iridiums last 100K miles.
Not surprised you getting poorer engine performance at 40k miles on the IK20L spark plugs. Yes, they could be fake but ..... A lot of us think that all iridium plugs should last 100k miles, but apparently not so. Denso here below says service life of their IK20L plugs is 30k miles.
https://www.densoproducts.com/denso-5358-ik20l-iridium-power-spark-plug#productSpecifications
The link also says the ground electrode is plain old nickel. It is not padded with a platinum or iridium puck, hence the visible erosion of the ground electrodes in your pictures.
Kudos to you for your choice...
Some iridiums have platinum or iridium on the ground electrode, and can last 100k miles. Others only nickel material for the ground electrode, and longevity is 30k-40k miles only.
https://www.densoproducts.com/denso-5358-ik20l-iridium-power-spark-plug#productSpecifications
Denso specs their 5358 IK20L for only 30K miles.