Sorry to reopen an 'old' topic, but I thought I'd do some testing to see the difference between hoppers that are searching for an inventory, and hoppers that aren't.
First, I created a superflat world of one layer of hoppers:
40 fps average - a third of my norm.
Then I tried with a layer of furnaces above the hoppers:
120 fps average - normal for me.
Then, just to check, I tried a layer of stone above the hoppers:
40 fps average, again.
From this, I gather that having an inventory above a hopper completely eliminates practically all lag.