SUMMARY
Downloaded and installed Krita version 4.2.8 and opened a new document. I use by tablet to both zoom and pan with the bottom button on the pen set as middle click with the first button on the tablet set to Shift, the second to 5, and the third set to Ctrl.
When holding down the bottom button on the pen and pressing the third button on the tablet (MMB+Ctrl) there is a significant lag in zooming where nothing will happen for about a second and then the zoom happens. This only happens with the pen and using MMB on the mouse will zoom in and out fine. Likewise the mouse works fine with the tablet Ctrl or the one on the keyboard and the pen will also lag with either Ctrl.

I performed a full uninstall of 4.2.8 including removing all the files from AppData and reinstalled 4.2.8 with the same result. Did another full uninstall and reinstalled 4.2.7.1 and the issue was no longer present.

STEPS TO REPRODUCE
1. Install Krita 4.2.8
2. Set tablet for MMB on pen button and Ctrl on tablet button
3. Attempt to zoom using MMB+Ctrl with pen and tablet

OBSERVED RESULT
There is an extreme lag between the time that the action is taken and the moment that the program registers that the action is taken

EXPECTED RESULT
The document should smoothly zoom in and out.

SOFTWARE/OS VERSIONS
Windows: Win10 v.1903
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
GPU: NVidia GeForce GTX 960M
Tablet: Wacom Intuos Pro PTH-460
Verified that all drivers are up to date for both GPU and tablet