Displaying item level makes bank sort very slow.
I ran an experiment where I removed the first item from a full 30-slot bag and then sorted the bank. I timed it on my phone, so my accuracy not much better than a second.

Sorting the 30-slot bag with all UI mods disabled is close to instantaneous.
Plain Bagnon with no extras took about 1 second.
Bagnon with Bagnon Item Level took 9-10 seconds.
Bagnon with Bagnon Garbage took under 2 seconds.

I also tested a couple of other mods that I was suspicious of:
Skillet is a dog and took 22-23 seconds.
Scrap with everything enabled took about 6 seconds.

Labeling all slots from scratch when the Item Level addon is enabled takes less time than relabeling the part of inventory that was sorted. I suspect that the problem is that Blizzard sorts bags by swapping items pairwise and that all of these mods are updating the slots on every swap, even though it is not displayed until the end. Blizzard could probably fix this by disabling the callback hooks until the sort is completed. A lazy evaluation of slot updates in Bagnon could make sorting less painful.

Thank you for fixing this!