Please post the output of lshw -c display from the terminal (ctrl+alt+t)
A few things for you to try:
- See if any drivers are listed under Software Updater -> Settings -> Additional Drivers and try them out.
- Look at the post regarding discrete graphics here. If your BIOS doesn’t have the feature, look at the last post on that same thread on how to disable hybrid graphics.
- See if any driver is listed for your card under https://www.amd.com/en/support and post the link back here.
- See if any of the drivers work on Synaptic - search “radeon” when you’re on it.
- See if the lagging happens when specific applications are open (open Utilities->System Monitor->Processes tab and sort by highest users) and let us know which ones.