I think I did not state the problem clear enough.
The solution given here supposes that the fontsize is known beforehand.
However, what I am doing is calculating a font size based on the current one and than switch to that size.
It is part of a fontsize changing mechanism inside an xmlsetup. In short what I am doing is:
\scratchdimen=\the\dimexpr\bodyfontsize\relax
\advance\scratchdimen 'somevalue' \relax
% \definebodyfontenvironment[\the\scratchdimen]
\begingroup
% must keep the change local
\switchtobodyfont[\the\scratchdimen]
...
\endgroup
The commented out \definebodyfontenvironment kills a lot of the font messages but I fear that each the time the same work is done as without it. The only benefit being the suppression of the font messages in the log ;-)
It might be impossible to accomplish this efficiently -- that is execute the font calculations once only for the given size. But at least I would like to give it a try.
Hans van der Meer
On 14 May 2016, at 20:00, Wolfgang Schuster