I have had this issue before as well, when you export the model select a lower setting for the part in freecad if possible.
with onshape the export tool gives a option of fine medium or coarse. I use the coarse setting if I want less triangles on the model in stl format.
you can also try lowering graphics settings in solidworks as well if you have not already.
FreeCAD should be able to use STEP format to export as nurbs surfaces (if the model was created in FreeCAD), without having to use a mesh format like STL.
Meshlab and Blender have decimation, so you could reduce the # of triangles and then import the mesh into SolidWorks. SolidWorks STL import can only handle about 15,000 triangles as a Solid Body, or somewhat more as a Surface Body.