In addition to the fast document generation, the SpreadStreamProcessing can be adapted to import documents significantly faster than SpreadProcessing.
The UTF8 with BOM encoding should be used to properly preserve the different characters. Available in R3 2017 Official Release.
Provide a way to measure text on different platforms so the user can pass the cell value and SpreadCellFormat and get the size of the text in pixels. This way we will be able to implement auto-fit to width functionality (AutoFitWidth), at least for the .NET Framework implementation. It will be able to measure the width of all cells in a column and set the value as column width in pixels.