Weird behavior in Sales Orders

Hi experts,

First of all I hope I created this thread in the correct session. If not feel free to move it to wherever it belongs.

One of our clients started to have performance issues when selecting a particular Item in a Sales Line ... Let's name this item XYZ.
As soon as they validate the Quantity the whole Sales Order page is not responding for 5-15 seconds, and after the initial freeze the whole order pages responds really slowly. XYZ is used in nearly every Sales Order (40k SalesOrders a Year)

SideNote: we dont delete Sales Orders when they're done - just mark them as done.

Ok. Now, heres the weird part:
The Item XYZ is slow for every User EXCEPT for users with the SUPER Role. With Super it works like a charm.
I checked the item Card and all the other linked Tables for the item XYZ - there was nothing special about it.

Next i started SQL-Tracing a Non-Super User and found this:

https://us.v-cdn.net/5022383/uploads/FileUpload/ee/75eb944a2a3bd470a3c788d1ddf81e.png

Looks like we are sending a request for every SalesOrder in which the item is used! (A13/.... are SO-Numbers).

For me this looks like the FlowField "Qty. on Sales Order" is somehow broken. I compared Objects and VSIFT views with a standard Version of 2013R2 but wasn't able to find any difference.

I already looked trough Cumulative Updates to find something about this issue, but couldn't find something.

Please excuse my english!
I would greatly appreciate any help, if i can provide you with more information, let me know what you need to know!