I am currently using expect on windows, only to launch
a console app while preventing a console to show up.
I just issue a

spawn scilex.exe -f file.sce

then the application opens some graphical windows. The
problem is that the application takes several seconds
to show up.

When I launch it with exec, i.e.,

exec scilex.exe -f file.sce then the application
shows up almost immediately.

How could I speed up the application launching when
using spawn ?

Thanks for help

So, maybe try running exec first, then spawn second.

[Full disclosure: I've used exec, but not spawn.]