I'm also noticing the lag. 4-10 seconds between each selection on the canvas. I didn't try the element tree option. When I started using SD there was no lag, but as time went on I noticed the QT* thing expanding in size.
The "QTwebengineproccess.exe" is the largest consumer of memory out of all my apps. Site designer is not too bad. I do believe that the QT* is what is used to create the gui and more that SD relies on.
please see the attached file.
Also, the canvas I was working on was just a header, list container and some paragraphs.
I opened a support ticket for you within your account regarding this along with a possible solution.