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If this was mechanical you'd put it down to something fouling the cable release. Obs with Di2 anything which prevents the jockeys moving outbound must be in the rear mech and this does sound like a 'mechanical issue' rather than electronic (motor or software). Have you given it a really good clean off the bike to get into all those nooks and crannies? Perhaps a twig or leaf that could be slowing down but not preventing the release?
It is also possible to change the parallelogram part so it would be worth checking that is nice and secure. possibly a jockey wheel lose or worn?
Alan_tbt
Recently my bike with di2 has been slow to shift down the cassette (to the smaller cogs), but only for 3 specific cogs: 2nd largest to 3rd, 3rd to 4th, and 4th to 5th. Shifting up the cassette is very quick. Shifting down across all other cogs is fine.
The chain was just over 50% worn so I replaced it. However, the issue persists, and I've now realised it does it across 3 different cassettes (2 wheelsets and turbo).
The mech hanger doesn't appear bent, and is only about 18 months old. The derailleur appears to be tight on the hanger. I've had the bike for 3.5 years now and shifting has always been perfect until a week ago.
Any ideas to help me diagnose and fix the issue? I'll plug in eTube and update the firmware if it needs it later. However, I'm struggling to think why/how there could be an issue with just 3 downshifts. Typically a bent hanger, or needing to micro-adjust, would result in the shifting being off over the whole cassette.