My system: 3 monitor set up with RX480/8gb Gigabyte reference GPU.
The releases between 18.5.1 and 18.9.1 were a problem for me with mouse lag. 18.9.1 fixed my issues and 18.9.2 seems even better with the Radeon app being more responsive.
Keep in mind that the 18.9.1 release notes reported this issue as fixed so could be you have some other system conflict occurring.
"Cursor or system lag may be observed on some system configurations when two or more displays are connected and one display is powered off."