It was fast and now slow?
Any changes in internet service?
Have you added or changed any other services that seek the internet (music, surveillance camera's, on line ordering, office computers, gaming machines, Public WiFi)?
Any vendors of such equipment/service been onsite lately troubleshooting issues?
Are you running Anti-Virus on the back office computer?
IP conflicts? Is Micros router being shared with other devices?
Windows firewall?
Is it slow 100% of the time?
Have you changed any printer routing options that determine where the credit card slips print?