This can be a number of things. From a screenshot it is only possible to give very general advice. As it is "only" a parameter the system's structure shouldn't change. Therefore it is likely related to dynamics of the system. I would take a look at the solver log first, starting with the number of computed steps or the step-size. If that changed in an order of magnitude of the slow-down you experience this is a good starting point.
Another issue could be chattering, which could be judged with knowledge about the number of events that happened. These should also be visible from the solver log.
Some additional information that would help:
- Which Modelica-Tool are you using?
- The model itself (with which all of the below can be generated)
- The solver log (before and after the parameter change)
- The translation log incl. system properties like the sizes of equation systems (before and after the parameter change)
I'm afraid without any additional information you will not get utterly helpful advice...
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