I found an interesting parallel between this series and Parades End.
They are very different programs obviously, which I would think would appeal to different audiences. The only thing they have in common is both are period dramas set in the UK.
But they met a similar fate. Parades End debuted to about 3M viewers, which is high for BBC2. But it lost 1/3 of its audience on the second night (it was five parts total) and never recovered.
JI lost 1/4 the second night and has lost 1/3 by the third night.
The complaints? Exact same: mumbling actors, slow pace, hard to follow.
Of course Parades End got better reviews, but to be fair JI got very strong reviews from pre-screeners. After the Sound Scandal erupted the reviews were mixed and most of the coverage was on the sound/mumbling issue alone, the blame game and the BBC.
Anyway, I thought it was interesting. And from what I remember the viewers were not enthralled by Parades End as much as the critics were. Certainly not the ones that stopped watching, I imagine.
I watched one episode of Parades End. I was impressed by Rebecca Hall and thought the acting was good overall, although Cumberbatch didn't appeal to me in that role. I think he's a bit overrated.
But I didn't come back for another episode, I admit. Not because I thought it was bad but just because I could not get invested.
I think critics would be more inclined to like that series anyway. For one, there was no "sound scandal," just a loss of viewers due in part to mumbling. There were complaints but no media frenzy. And the source material is just more high-minded and sophisticated, which appeals to critics.
Personally I liked JI better. But the point they both struggled with the same issues that ended up alienating about the same percentage of the audience.