From https://www.graphviz.org/pdf/dotguide.pdf:
dot also provides access to various parameters which play technical roles in the layout algorithms. These include mclimit, nslimit, nslimit1, remincross and searchsize.
Where does generating the graph take most of the time? - #2 by smattr sheds some information on dot tuning (stackoverflow might have more)
Two specific things:
- you are using a 7 year-old version of Graphviz. That probably isn’t helping.
- you might try changing your splines setting. Ortho is somewhat problematic.
If you upload your source, we might be able to give more specific guidance.