You may be aware of this example already, but the spreadsheet siag can use either guile or siod as an extension language. It runs considerably slower when using guile. I know that several people have written against using siod in such a way that you would think that it is unsuitable for anything. Yet, somehow it works well for siag. The answer for LyX may have more to do with how much you expect to implement in the extension language. John Jones jj(a)asu.edu