Hello.
I notice a rather strange behaviour in Lightroom 4.3 in the Develop module: it appears that the Develop preview ("zoom to fit" setting, I guess that's what 99% of people are using when using the global adjustments) is only updated when moving the mouse pointer AWAY from the slider that controls the adjustment (it doesn't matter if it's Clarity, or Highlights, Shadows, whatever).
I happen to often click somewhere on the sliders range, say somewhere in the middle between 0 and +100, to get an idea what a Clarity ~50 might look like. If I keep the mouse pointer hovering over the knob, the Develop preview will look pixelized/blocky, and not update to a more refined preview.
Only if I move the mouse pointer AWAY, the preview will be updated. It's as if the rendering is delayed until you're really sure you want that setting, perhaps to improve responsiveness/performance? This didn't happen in LR 4.2.
I tried this with a NEW catalog and only imported something like 100 photos to it, and no plugins - the behavior stays the same. I uninstalled the latest AMD/Catalyst graphics driver and let Windows 7 (64bit) "fall back" to a default driver - again, behavior stays the same.