What I find time consuming (annoying) is not the products you can find, even if you are entering 3 prices, it is looking for an elusive item or an item they don't carry. I have wasted time searching for "natural" or "organic" items (even toothpaste) that may be in a healthy living section all to themselves or blended with other like items (amongst other toothpaste brands). However, I was never given the chance to do these on a rotational basis, where you could benefit from these lessons learned.
The employees I encountered kept wanting to help me. Ultimately, it may have been their way of seeing what I was doing. Obviously less of a problem at a large retailer and more so at a specialty, lose your whole paycheck kind of store. Also, at a large grocery store, they had an area for dining. I could recharge my phone, get a drink and just take a break. Depending on the number of items, allow yourself to break it up in more than one visit.
That said, I would do it again and I had 750 items. I was entering my information into the Missions app. If I recall correctly, you can enter prices off line, then access wifi later to submit (or sync).