Are all devices slow or just the computer that received the graphics card?
Did you try reinstalling the graphics' card drivers and verify the configuration?
If just the computer then I would open it up and inspect all connections and cards. Maybe something came loose during the install.
If all devices first check computer as suggested and then look over the rest of your network.
Use Task Manager, Resource Monitor, and Performance Monitor on the computer(s) to determine if some resource is being highly consumed by a process, service or application.
Need to narrow down the possibilities.