After experiencing Lag spikes on our server(vanilla) since we launched 1.17 (new world) we finally had an idea and tested it. 

The Idea is that opening item containers causes a lag spike as the server gets the information for the items within. We've noticed similar things with all blocks that can have items in and villagers but we didn't test everything.

Test: everyone online, 7 people, stood in the same area. Lag was extremely low. We then simultaneously opened an item container. 6 Shulkerboxes and 1 barrel. All containing items. We closed the inventory as soon as it opened and re-opened it.

Extreme lag started immediately.

We lost over 2k ticks and the server didn't even calculate the loss until we had stopped.