Every append ends up calling nsTableRowGroupFrame::GetRowCount (from nsTableRowGroupFrame::AppendFrames, so that we can convert to row indices for cellmap maintenance, looks like). GetRowCount walks our entire child list.... Not sure what the best solution is here, though. Adding members doesn't seem warranted; this is O(N^2), but with a _very_ low constant, really.

does that appear on any jprof?

Yes, that's how I found it. It was about 5% of the total time for loading a rather large table (hence my comment about low constant).

there is http://lxr.mozilla.org/seamonkey/source/layout/html/table/src/nsCellMap.h#476 which holds the right information so we need just to drill through the table frame to the table cellmap down to the right cellmap and get the info.