One possible reason can be the increased memory usage - particularly if you're using mismatch (you end up with a lot more models, as each device has its own model).
Because of the extra memory, perhaps it is swapping?

Are you using MMSIM60? If so, that should help with memory consumption (the new front-end is much more memory efficient in these situations).

There's not normally much benefit in doing the nominal run in my opinion (except as a sanity check?).

Regards,

Andrew.