I am interested in talking, as I work on ethercalc.
The big issue I have seen is formula calculation speed. I understand render speed is also an issue. I don't know your use case, but the use cases I see tend to have formulas. It is hard to makes formulas fast enough with JS.
The main speed problems with ethercalc are loading the data from the server and calculating the formulas. I did strip down the code to remove these problems to make web apps work.
Web apps as simple as spreadsheets. My aim is to make it easy to add a UI to a spreadsheet. Spreadsheets often have a UI bottle neck. I have seen many spreadsheets that would be much more valuable to business if it had a UI, as more people could use it. So I added UI formulas to ethercalc http://sheet.cellmaster.com.au/examples