Somekind of lag with strongboxes

It's not uncommon for me to see strongboxes kinda lagging in popping out items after clear up all mobs.
But the worst part is even some of the extra mod is lagging - eg, explode.

I was at a jeweller box, cleared all mobs, and it's silent for 3 seconds... nothing happens. I stand beside the box waiting for loot and booooooom!!! R.I.P.