There is a fascinating way of coming to the idea that the problem is almost always testing. Here's a little story of something that has happened to me many times in many organizations, and was recently inspired to think about. Maybe it is because it is almost Christmas. :)
Speaking in metaphors, the box with Christmas Ornaments inside.
Once upon a time, there was a product owner who ordered a Box with Christmas Ornaments. As product owners go, they diligently logged into Jira their Epic describing acceptance criteria clearly outlining what the Box with Christmas Ornaments would look like delivered.
The Developers and the Testers got busy with their respective work. Testers carefully reviewed the acceptance criteria that was co-created, and outlined their details of how testing would happen. Developers outlined the work they need to do, split the work to pieces, and brilliantly communicated to testers which pieces were made available at each time. Testers cared and pinged on progress, but when things aren't complete, they are not complete.
The test environment for the delivery was a large table. As pieces were ready from the Developers, their CI system delivered an updated version into the middle of a table. The Box with Ornaments was first a pile of cardboard, and everyone could see it was not there yet. But as work progressed, the cardboard turned into a Box, without the Ornaments. As per status, pieces were delivered (and tested), but clear parts of the overall delivery were still undone.
Asking the status and wanting to be positive, Developers would report on each piece completed, and the Box on the table looked like it was there. It was there quite some time. Asking status from testers on testing, they would learn that testing was incomplete, and it was so easy to forget that there are scenarios that required both the Box and the Ornaments to make sense of the final item, even if we could and had tested to learn about each individually.
The product owner, equipped with their Epic in Jira looking towards the table concluded:
Things get stuck in the process. They are long in an intermediate stage. It feels like they don't care about delivering me my package, they just leave it lying around for testing.
It's not like they ordered the Box without Ornaments. Yet they feel it looks ready enough that putting the Ornaments in is extra wait time.
To achieve flow of ready to the hands of whoever is expecting, optimizing developer time between multiple deliveries really does the negative trick. Yet we still, in so many cases, consider this to be a problem with testing.
I know how to fix it. Let's deliver it as soon as developer says so. No more Testers in the place you imagine them - between implementation and you having that feature at your hands.
A better fix is to deliver the empty box all the way to the customer as it is ready, and carefully think if the thing they really wanted was the Ornaments, and if another order of delivery would have made more sense.