We've been back & forth on this numerous times. Every time I try to update our item database using the IMPORT tool, the lack of speed is worsened by the fact that Method somehow creates a duplicate/ghost UnitOfMeasureSet entry, haphazardly, and for no apparent reason, for random products (in this case, so far about 763 of them -- last time it was about 40,000 of them). This results in a situation where the UnitOfMeasureSet text is correct, but is somehow linked to an ObjectID that is NOT the ObjectID associated with the displayed UnitOfMeasureSet. Last time it was EACH. This time it is BOX. Woe is me.
We are solving this by eliminating the UnitOfMeasureSet field entirely and replacing it with one of our own that does NOT sync back to QB. In fact, we're doing this with almost every piece of information in our database as I have completely given up hope in regards to the QuickBooks SDK and its ability to properly synchronize data without chewing it up and screwing it up.
In the future, your development team may want to seriously consider taking a look at why this is happening. I had assumed it had been fixed after the horrible situation we found ourselves in last time, frantically working with support to fix more than 40,000 messed up products. Nope. Still broken.