I had an old system with about 20k stacks of stuff and built a new system that are modules i can attach.
When transfering from the old to the new with a inventory hopper it works like a charm with a filter item that are set to transfer everything but a juk item. But when my new module got full the mspt took a really good hit, dont remember the exact mspt but my tps was about 1.1 or lower so had to download my world from server start it up locally and then i could add a module and now it works as it should again.

I got this error before to but not sure what hopper it was that time i just teared down all hoppers (10:ish) and it worked after that so i guess it was trying to push when an inventory was full also that time.

And i love your mod so much. Would absolutely say its the best inventory mod i used at least :)

Seems to be the same issue when my old storage got empty then mspt went up right away until i stopped the hopper either by redstone or removing it completely then i get my mspt back.

Tried the same now and when trying to fill a chestmonster with a connector and the storage got full i got 150 MSPT instead of 1.5 when just connecting some more chests. Have images if needed.

Please just check how the mod handle a full storage compared to onethat is not full.

In my example i added 105 chests to the storage witch should make it slower but it did exactly the oposit when full i got like i said crappy mspt but when adding 105 chests i got 1.5 in mspt when transfering from one chestmonster to another. I could i guess record a video if needed to show you

First video ever made by me :)
https://streamable.com/4xk1fx

[edit] new video