I recently had the opportunity to try a new Samick SIG-57, and I was pleasantly surprised. It had a nice, mellow sound that easily rivaled some of the more expensive pianos in the shop. But I had a real hang-up with the action. I tried playing some Mozart sonatas, and the keys simply could not handle the quick repetition that some of the faster movements required.
Is this a manufacturing flaw in the piano, or is it something that a skilled technician can fix?
Yes, but it might not be easy or inexpensive. Samick, at least back when we were dealers, used wippens with what is called a "wippen assist spring." This spring acts to lift the wippen and reduce, or eliminate, the need for key leads. The problem is that the springs are not of consistent tension and, as it turns out, the action works better with at least some lead in them.
Our practice was to pull the action stack and readjust the springs so they just balanced the wippen. This removed a lot of their tension. Then we balanced the keys in the normal manner (although they still didn't need nearly as much lead). This was not a perfect solution but it did make the actions a lot more controllable and responsive.
It did add to our cost in the pianos but they performed enough better that we figured our sales offset the cost. Besides we much preferred how the actions performed after our little modification.