Hi cavac, and thanks for your effort.
I actually combined your idea with Marshall's and ended up with a blazingly fast simple Tk::TableMatrix based spreadsheet in which I replace the cell over which the mouse pointer currently hovers with an actual editable widget. I had to rewrite quite a few GUI mechanisms (binds, binds, binds...) but some 10 hours later I am pretty much back to where I started, only with a responsive GUI.
Only things I miss are autowidth of columns (my data differs quite a bit in size within the columns) and balloons. I guess you just can't have it all. Maybe I could attach a balloon to the cell I am currently hovering over but that would require me to first replace the TableMatrix cell with a (Tk::Label) widget, like I do for editable cells.
Thanks!