To repro, Fill 5 shulkers full of items, place those five shulkers into a hopper. Face that hopper into an empty shulker. Next steps prove that it isn't client side
Large amounts of lag will occur. The easiest way is to go to another dimension or an area a long way away (as a way to show it is server/host side not client side), and place gravity blocks while the hopper with 5 full shulkers are loaded on a second players device. This will cause entity rubber banding and block placement lag server wide.
Empty or partially full shulkers seem to cause less lag, this leads to the assumption that the data in the shulker is a contributing factor to the lag. Based upon this, the assumption is that the data in the shulker is put a transfer request then canceled ever tick,
https://youtu.be/eTPg_hXs8hM world file is attached.