Dummy derivative should be added only when there are no other derivatives.

[Adrian Pop]

To summarize:

• add dummy state only if there are no other states
  - in that case, use der($dummy) = 0
  

Adrian, can you do this and check with the slow example you got, as well
as with a fast-changing, purely algebraic model like:
{{{model fastAlg

Real x = Modelica.Math.sin(time*1e3);

end fastAlg; }}}

Cheers,
Francesco