If you're using an iPad, please check this guide for help.
Try the following things:
Restart your browser
Make sure you're using a recommended browser (you can find the list above)
Close all other browser tabs
Close all other applications
Restart your computer
Create a new canvas with 1920 x 1080 size and see if the problem happens there as well (sometimes large size or a large number of layers can be the cause of lag)
Turn off the "stabilization" feature (it helps to draw smooth stokes, but sometimes it might be perceived as a lag since it delays the time between pen or mouse move, and the actual drawing appearing on the screen)
If nothing above helps and you're still experiencing the lag contact our support team including the following information:
your software & hardware specification
link to canvas when the issue is happening
browser performance report (read here to learn how to prepare it)
(optional) recording presenting the lag