Yes yes yes. I tend to waste a large portion of my flaming humbaba time navigating my inventory (and how long that takes depends partly on framerate and lag).
In the meanwhile, I have a system to grind efficiently you might like to try. I put the grinder next to my humbabas, and pick up exactly the number of rocks I want to grind from my SDB. I drink the humbaba, then click the grinder and choose "grind". The grinder pops up the rocks available. I click the rock and because I've got exactly the right number of rocks in inventory I can just click "all" rather than having to try to enter a number. Very little humbaba time is wasted.
I can do 250 rocks every time without rushing my clicks. I've read of players doing 300 and even 400 and I worked out the 250 a while ago so YMMV.