This may be an issue with packing or posting your data & sales. When was the last time you backed up the data? After the backup, the software compacts the data so it runs more efficiently. Also, posting sales moves some records from databases that are always open when you are in the Point of Sale screen. If you have never posted, these files are probably really big and take that much more time to access/write to them over a network.

I hope this helps. If not, report back to this thread and we'll see what else we can do.