Delayed Processing when Merging Unprocessed Sales Order

Since Upgrading from Sage Evolution 7 to Sage 200, anytime I merge Several Unprocessed Sales Orders into 1 invoice (one customer/timeframe within 1 month.....number of orders immaterial - same time for 2 orders as well as 20 orders) , the transaction takes well over 15 seconds at a minimum, and during this time it completely hogs server resources and Sage hangs and no other process or function can be done on the system.

prior to the upgrade - this function used to take less than 1 second , even when done by multiple users.

We have tried restoring the data to higher spec servers, running even the most recent SQL version but there's no improvement

Any suggestions how to go about resolving the problem?