Technically, we actually handle slow objects in one of two ways:
If the object is standalone (not part of a linkset) then we make it Phantom.
If the object is the root prim of a linkset then we make the entire linkset Phantom. That's because in Second Life and OpenSim, it isn't possible to make only the root prim of a linkset Phantom.
If the object is part of a linkset, but not the root prim, then we change the object's "Physics Shape Type" to "None". This has the same effect as making it Phantom, but without having to make the entire linkset Phantom.
How to Make Complex Objects Non-Phantom Again
If you received an email that notified you that we made some objects Phantom, and you don't want them to be Phantom, then you need to do the following:
Find the slow objects (they are listed in the email that you received).
For each object, edit its physics mesh in order to make it simpler (use fewer triangles). For objects that are themselves meshes, this may be as simple as re-uploading the mesh, while selecting a simpler Physics mesh in the upload dialog.
Remove the Phantom flag from these objects. (Or, if the object was part of a linkset but not the root prim, then change the object's "Physics Shape Type" from None to Prim.)
We check for slow physics meshes every time a world starts, so if the new physics mesh is still too complex then we'll notify you about it the next time the world is started.
If you want to use this trick then follow this procedure:
Remove the Phantom flag from the linkset.
Unlink the linkset.
Re-link the linkset, but this time make sure that the root prim isn't the slow object.
If you complete these steps, then the next time your world starts our system will only change to Phantom the individual slow objects that are in the linkset, instead of the entire linkset.