There are two related ways that i know to speed this up dramatically. Both have to do with the fact that the main spreadsheet area doesn't actually have to use "real" widgets. In both cases, you just draw the data and any visual sugar as graphics.

The first way is the classic spreadsheet way: You have one long editable field near the top of the screen. If you select a cell (calculate from the mouse click on the graphics which cells this would be), put its value in the edit field. Every time that value changes, redraw the appropriate part of the graphics, so the user thinks they are simultaneously editing the actual value in the table.

The second way is one i have used before for HTML/JavaScript based tables (i think DataTables does it that way as well): again, use a generated image as the spreadsheet proper. If the user selects a cell, just spawn in an appropriate widget for editing on top of the image. When the user is done, remove the widget.

The main advantage of doing the spreadsheet as simple graphics is speed and memory usage: All you do is print some text and paint some lines. Doesn't need to bless a gazillion instances and register a gazillion callbacks. You basically only have two callbacks: Mouseclicks (selecting cells) and scrolling.

For moving between cells with the cursor/tab keys, you would just get those from the one and only editable widget.