Currently Valentina uses font for layouts which are not compatible with pen plotters. (fonts are not optimized and it takes a minute to print two letters. Normally it takes two minutes print whole layout including all inscriptions) Pen plotters are most commonly used to print pattern layouts in production environment.
To use single-line font for layouts. (those fonts are optimized for engravers, plotter, etc.)
Below there is an article on how inkscape extension makers solved this issue. Maybe it will be helpful