I can't really test it scientifically, but what I can see and feel does not indicate much input lag, if any at all. I do know that Hardware.Info mentioned their input lag testing device had strange results and they asked iiyama for an explanation, but nonetheless it still received an excellent award (without input lag results that is, I think that says something).
IF there is 25ms input lag, I can't notice it at all.
Also, the fact that Direct Drive Mode is not present on this model does not automatically mean the B2 has input lag issues. It might use better internals or a better panel which made Direct Drive Mode redundant. I think the reviewer thinks that because DDM is removed, the monitor has input lag issues like the B1 would have without DDM. I'm pretty sure this monitor differs quite a bit internally from the B1 (the HWI review confirms it judging by the vastly different results from B1 and B2), so I'm pretty sure it's just not needed anymore.
I made one mistake earlier by the way. The default Overdrive setting is 0, it ranges from -2 to +2 and with an off setting. So it's not off by default.