“Diablo 4 Associate Game Director Joe Piepiora clarified the situation a bit on Twitter, stating that Stash tabs are memory hogs. Whenever you encounter another player, be it in Kyovashad or out in the wild battling enemies, the game loads not only the physical character and gear, but also that player’s entire Stash.”
there we got the CPU 45 degree to 90 degree spike for a nanosecond every now and then? i always felt something is wrong within the code of the game 
btw: the first thing i do when making an ARPG or any other LOOT based game, is to make sure that its easy to provide enough stash to the people as this is one of the most important things in that genre, to make sure there are no hard technical limits^^