Well. I thought I'd get my hands dirty and take the xnode apart. You are right. It doesn't just call moveblock. In fact, it doesn't call it at all!. It calls "GetValueByPointer" in "lvimptsl.dll" (that might call moveblock). But, more importantly (as you surmised), it does this in the UI thread (presumably because the dll isn't thread-safe). So. You're on a roll , That's the reason it's so slow (and totally unusable for 99% of my apps).
But your suggestion works a treat