I try to import an xlsx sheet into a TadvStringGrid using AdvGridExcelImport.import(xlsx filename)
and despite the xlsx sheet contains only a few used lines (35), Import is very very slow (more than 5 mn)...
Tracking that point i remark than the rowcount property of TadvStringGRid after import was 1 048 575 so the entire number of lines of Excel Sheet
Is there a way to import only useful lines ?

Sometimes files have indeed some garbage in a row that is way after the useful data finished. You can normally spot it by looking at your scrollbar in Excel. A normal file will look like this:

But your file likely looks like this:

You don't need to have actual data at row 1,048,575 if might be just a row that has a higher size, or some invisible format, but this will cause FlexCel (and Excel) to load all those rows. FlexCel will load them very fast, but then when transferring them to AdvStringGrid it will indeed get slow.

About a property to ignore empty rows (and columns), it was in our plans but looking at the code I see we never managed to implement it. But we should, and next release of the Bridges will have this property. It is sadly at least a week away because unrelated stuff we need to finish, but next release will have an option for that.