Thanks, this is exactly the kind of information we're looking for.
We'll take a look at doing some more work with the mouse to make interaction easier.
In our initial pass, we didn't consider doing partial line/object erase, we'll take a look and see what that might take to implement.
Good point on the alt+o, the tool hotkeys were basically just slapped in as they were added. We will rebind them to make them easier now that we have the features completed.
Can you describe what was lagging? What browser are you using? Currently, we know there are some performance problems with dragging objects but placement has been quick in our testing so far.