I guess I'll try limiting current to only 1 day? Staff are saying that it gets slower as the day goes on (as they enter more invoices). Each day the cycle repeats as some are cleared out of current and more are added.

The only complaint I'm getting regarding going to history instead of current for everything is that when they "restore" instead of "copy" an invoice, it seems to do the same thing except it won't copy the serial numbers.