System cursor:
Setting > Configure krita > General > Cursor > Brush Cursor Icon for system cursor.
Personally, I prefer “small circle”.
Quick toggle brush outline:
Setting > Configure Krita > Keyboard shortcut and search “toggle brush outline” and assign a shortcut
OR
Setting > Configure Toolbar, search “toggle brush outline” and add it to the toolbar.
The action will show as plain text when adding to toolbar, so I prefer using keyboard shortcut.
Edit: It reminds me Ralek’s reply to another post that also asking brush outline lag. [Solved] Stylus kind of Lags in Krita? - #11 by Ralek (#11 and #13)