The current data storage method, Will load into memory every player that's been on the server since the plugin's data has been wiped. Which becomes a large issue if your saving their inventorys.
Consider using seperate data file for each player, then loading data only as needed. there's a guide on how to do so in documention. This frees up memory and gets rid of the lag and processing trying to go through all the player data to find what's needed.
The sooner you make the change, the less hassle it becomes. I know most plugins fail at using the better method.