I am wondering why it takes two to three years to build this tiny little international docking adapter IDA 3? Every year 4-6 Soyouz spaceships are built.
The IDA was never meant for mass production: they needed just 2 for the ISS, and they knew long beforehand when they needed them. So they knew they could spend 2 years building them without impacting the schedule. They built the things by hand, took their time testing them etc. This is labor-intensive, and labor costs money.
To reduce the build time, you have to spend money too: build tools and jigs, design factory processes (e.g. an assembly line), design the IDA for manufacturability, etc. So you have to decide: spend money on labor, or spend money on R&D so you can make the module cheaply? This decision depends on how many modules you need. In this case, only 2. That's an easy decision to make.
Of course, then the launch of IDA 1 went wrong, and the 2-year production time bit them in the ass.
By the way, the fact the Russians can produce 6 Soyuz per year doesn't say anything about how long it takes to build one Soyuz: it could still take 2 years to build one spaceship, if they build 12 of them in parallel to get the production rate they need.