A lag spike occurs when breaking, holding, moving within inventory, or opening an ender chest containing a shulker box full with buckets of tropical fish of a known variant (e.g. Bucket of Silver-Teal Brinely).  The lag stops once the shulker is placed or is no longer selected in the hot bar.

The lag gets worse as the number of buckets and shulkers increase, i.e. the most severe lag will occur when opening an ender chest full of shulker boxes that are full of known variant tropical fish.  However, breaking and holding the ender chest does not result in lag.

A shulker box that is full of other aquatic mob buckets (including Bucket of Tropical Fish) will not experience the lag issue.

Picking up the full shulker feels like the game is loading 27 entities instead of 27 items in a chest.