Hi there,
Sorry to hear about the brush lagging in Photoshop.
Which version of Photoshop and the operating system are you working on?
You may try resetting the preferences of Photoshop and see how it goes: https://helpx.adobe.com/photoshop/using/preferences.html
Note: Make sure that you back up all your custom presets, brushes & actions before restoring Photoshop's preferences. Migrate presets, actions, and settings
Regards,
Sahil