It looks like one of the very recent post 1.8 updates has reduced hopper cart effectiveness dramatically.At multiple guardian farms we use hopper carts, with a layer of glass over top to collect drops. This is the least laggy way to kill mobs quickly and collect drops. Now the items just hang there as carts go by and only pick up a few items now and then. I've sat here watching a hopper cart go back and fort under an item and not pick it up. I've verified that the cart was indeed empty.