I have some custom items on my server. One is extremely high poly and the server lags when your character loads that item from the seller (note, not purchased but clicked on in list). If you then place it in the world, everyone lags as their client loads the object into memory I'm assuming. The problem you describe sounds like (for us), when a player logs out with that item in their inventory, upon login all players experience that lag once as the item is loaded into their memory...causing a server wide lag spike. New items coded the exact same way but low poly do not suffer from this. Important note, it only happens once per session. Not sure if that's your issue but hope it helps.