StellarMate does not use a commercial VNC server (requires licensing), just X11VNC. I found if you use RealVNC, it's pretty snappy and faster than using webVNC. Until release of next stable INDI/KStars in a couple of weeks, you can try the
Beta channel
Another thing that may help with Stellarmate responsiveness is making sure you are on 5GHz Wi-Fi. For some reason I can’t make it stick and so have to manually set Wi-Fi country to US and then switch to 5 GHz manually each time I boot up. Doing so gets better network throughout, at least for my setup where I am close and have no walls to go through.
I will try out using RealVNC instead. I have been unsuccesful in setting my Stellarmate installation to BETA, nothing happens and it does not seem do switch when I press the button.
But which firmware is actually on your Sesto Senso 2 right now? On mine I have the 03.03.07 and it does not show this bug in SM 1.7.5. Granted, it is not a fresh install, but an upgrade from freshly installed 1.7.0.
*edit. I think I may have misread your comment. But yes, do upgrade the firmware.
Are you running an older firmware that is still compatible with the old INDI drivers?
My just New Sesto Sense came with firmware 3.1.12 and will not connect at all! All attempts with any USB port and auto-teach enabled return nothing to the log. "Connect" button goes green, but nothing ever happens.
I have a fresh install of both Astroberry and libindi on RP Buster 64
Everything else connects fine, but not the focuser